| Term | Definition |
|---|---|
| ALP | The Astronomical League of the Philippines, the newest national amateur astronomy organization in the Philippines.
ALP was founded in September 2003 by James Kevin Ty, Edmund Rosales, Jun Lao, and Allen Yu. Its focus is on observation, imaging, and instrumentation. For more information, please visit the ALP website at www.astroleaguephils.org. |
| Absolute Magnitude | A measure of the magnitude of a star, defined as the hypothetical apparent magnitude if the star was moved to a distance of 10 parsecs away from earth.
Thus absolute magnitude is a measure of the true luminosity of a star without regards to the effect of its distance from earth. |
| Age of the Moon | The number of days, or fraction thereof, since the last new moon. |
| Albedo | The fraction of the light hitting an object which is reflected back from it. The albedo of an object is a number between 0 and 1. The higher the albedo, the more "reflective" the object. |
| Almanac | A publication containing the predicted positions of the planets, the sun, the moon, and other heavenly bodies, often compiled with practical information to users.
The official almanac of the Philippines is the Philippine Astronomical Handbook, available at the PAGASA Astronomical Publication Unit at the Science Garden in Agham Road, Diliman, Quezon City. |
| Alt-Azimuth Mount | A type of mount in which one axis of the telescope moves it in altitude, and the other axis moves it in azimuth. The popular Dobsonian mount is an alt-azimuth mount. |
| Alt-Azimuth System | A coordinate system on the celestial sphere using the astronomical horizon as its base circle. It is composed of the altitude and azimuth coordinates.
Altitude is the number of degrees an object is above or below the astronomical horizon, while azimuth is a measure of the direction of an object, counting the degrees eastward from the North point. |
| Annular Solar Eclipse | A type of solar eclipse in which the outer ring of the sun is remains visible when the sun, earth and moon are aligned. This occurs when the moon is far enough from the earth such that its disk appears smaller than that of the sun, thereby unable to completely cover it.
An annular eclipse will occur in January 26, 2009, but it will be partial from the Philippines. |
| Antoniadi Scale | A scale of seeing quality indicated by Roman numerals. I is the best, II to III is typical, IV is poor and V is extremely bad. The scale is due to Eugenios Antoniadi (1870-1944). |
| Aperture | The diameter of the main mirror or lens of a telescope or binoculars. The aperture is the most important specification of a telescope because it determines both the dimmest possible object that can be seen through it, and the smallest angle which it can separate two objects. |
| Aphelion | The position in an orbit around the sun which an object is farthest from the sun. It is also used to describe the time at which an object in solar orbit (such as a planet or a comet) reaches that position in its orbit.
Because objects orbiting the sun move in elliptical paths rather than perfect circles, all of them have both a perihelion and an aphelion. The earth is at aphelion at around July 4 each year. |
| Apogee | The position in an orbit around the earth which an object is farthest from the earth. It is also used to describe the time at which an object in earth orbit (such as the moon or an artificial satellite) reaches that position in its orbit.
Because objects orbiting the earth move in elliptical paths rather than perfect circles, all of them have both a perigee and an apogee. |
| Apparent Magnitude | A measure of the magnitude of a star as it appears to an observer on earth. Apparent magnitudes are expressed as numbers that increase as stars become dimmer, by a factor of 2.512 for each integral difference.
Thus a star of magnitude 4 is 2.512 times brighter than a star of magnitude 5. |
| Artificial Satellites | Man-made objects that are placed in orbit around the earth. Artificial satellites are used for various fields such as communication, weather forecasting and scientific research. The first Filipino artificial satellite is Agila II, launched in August 20, 1997. |
| Asterism | A group of stars that seem to form a pattern. They are distinguished from constellations in that they are not recognized as officially defined regions by astronomers.
The most popular asterism is the Big Dipper, which is actually just a part of the constellation Ursa Major. The Tatlong Maria is really the belt of Orion. Some asterisms contain stars from several constellations, such as the Great Square of Pegasus. |
| Asteroid | One of the many small or minor planets in the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter.
The largest asteroid is Ceres, nearly 1000 km across, but many are less than a 1 km across. The first asteroid named after a Filipino is Asteroid 6282 Edwelda, from Edwin Aguirre and Imelda Joson, both active amateur astronomers. |
| Astronomical Horizon | An imaginary great circle where the horizontal plane of the observer cuts through the celestial sphere. It is distinguished from the visible horizon which is where the land appears to meet the sky; the former is a fixed circle from which to base astronomical measurements, the latter may vary from place to place, depending on the contours of the earth's surface. |
| Astrophotography | A contraction of "astronomical photography". The practice of recording celestial objects photographically. Astrophotography often involves the use of the Single-Lens Reflex (SLR) camera due to its ability to let one manually set the exposure time. Today, astrophotography is a subset of astronomical imaging, which includes the use of other devices like CCD, video, and web cameras. |
| Atomic Clock | A highly accurate clock that uses the regular frequency of an atomic or molecular process to keep time.
In the Philippines, PAGASA maintains the country's Standard Time by an Rb/GPSCV-based Time Transfer System, simply called a Rubidium clock because it uses the cycles of a Rubidium atom to track time. |
| Autumnal Equinox | One of two points on the celestial sphere where the celestial equator and the ecliptic intersect. The Vernal Equinox is the point where the sun crosses the equator from south to north, while the Autumnal Equinox is where the sun crosses the equator from north to south. |
| Balmer Series | A series of spectral lines in the spectrum of atomic hydrogen. The lines are termed H alpha, H beta, H gamma, and so on, starting with the line of longest wavelength, which is at 656.3 nanometres. The lines become more closely spaced with decreasing wavelength, merging at the series limit of 365 nanometres. |
| Barlow Lens | A diverging lens used in conjunction with a telescope eyepiece. The Barlow lens increases the effective focal length of the telescope, which causes the eyepiece to yield a higher magnification. |
| Binary Star | Two stars that orbit around each other. Binary stars are gravitationally attached and travel together through space. If a star system has more than two stars, it is called a star cluster. |
| Binoculars | An optical instrument consisting of two small telescopes, mounted side by side, one for each eye. The relatively compact size of binoculars is achieved by the use of prisms to reflect the light internally.
The use of prisms also makes the image upright, rather than inverted as it is in a usual astronomical telescope. |
| Bok Globule | A small, almost spherical cloud of dark opaque gas and dust which shows up against a brighter background such as star clouds or a bright nebula. It is thought that globules represent an early stage in the star formation process. |
| Bright Nebula | A type of diffuse nebula that appear as bright patches of light. Bright nebulae are either emission nebulae or reflection nebulae, depending on whether they emit their own light or simply reflect light from other bright objects such as nearby stars. The bright nebula M42, also known as the Great Orion Nebula, is an example of an emission nebula. |
| Cassini Division | The apparent separation between the A and B rings of planet Saturn. This gap is about 2,600 km wide and is named after French astronomer Giovanni Cassini who first saw it in 1676. |
| Cataclysmic Variables | Variable stars that are really eruptive stars, i.e., stars whose brightness increase dramatically and suddenly in response to an explosive event. The term is applied particularly to novae and supernovae. |
| Celestial Equator | An imaginary great circle where the plane of the earth's equator cuts through the celestial sphere. It is exactly half-way between the north and south celestial poles, and neatly divides the celestial sphere into northern and southern hemispheres. |
| Celestial Poles | The two points where the projection of the earth's axis of rotation meet the celestial sphere. As the Earth rotates, the sky appears, to an observer on the Earth's surface, to rotate around the two points. |
| Celestial Sphere | The sky considered as the inside of a hollow sphere with the observer at its center, in order to describe positions and motions of astronomical objects observed in the sky.
Distances on the sphere are measured in angular separation and do not depend on how far away the objects actually are. Positions are described in either the Alt-Azimuth, Equatorial, Ecliptic or Galactic Coordinate Systems. |
| Cepheid Variables | A type of pulsating variable that regularly varies greatly in brightness, named after the group's prototype, Delta Cephei. |
| Cloud Cover | The percentage of the sky covered with cloud, expressed as a number between 0 and 1. A cloud cover of 0 means a totally clear sky while 1 means a totally overcast sky. |
| Comet | A relatively small, rocky, icy celestial body orbiting in the solar system which partially vaporizes when it nears the sun, developing a diffuse envelope of dust and gas and normally one or more tails.
The word comet comes from the Latin stella cometa which means "hairy star". The most spectacular comets in recent times are Hyakutake and Hale-Bopp, which appeared in 1996 and 1997 respectively. |
| Constellation | Any of 88 well-defined regions of the celestial sphere, and that are named after religious or mythological figures, animals, or objects which the ancients imagined them to form.
Originally, constellations were regarded simply as star patterns. In 1930, there was international agreement to define the boundaries of 88 constellations, so they can be used to specify stars and their positions. |
| Contact | In a solar eclipse, the moment when the limbs of the sun and moon appear to be just touching. In a lunar eclipse, the moment when the limbs of the moon and the earth's shadow appear to be just touching. |
| Crescent | The phase of the moon or a planet when less than half of the disk is illuminated |
| Dark Adaptation | A state in which the eye becomes more sensitive to faint sources of light as it gets used to the dark. During the period of adjustment the pupil of the eye increases in size to allow more light to enter.
Dark adaptation is essential for visual observations. It is instantly destroyed by any exposure to bright lights. For that reason, observers use dim red lights when reading star charts in the dark. |
| Dark Nebula | A type of diffuse nebula that neither emits nor reflects light. It appears as dark areas and contain no stars; in fact it blots out the light from stars behind them.
An example of a dark nebula is the Horsehead Nebula in Orion, which is visible only because it blocks the light from the bright nebula IC 434 behind it. |
| Deep-Sky Object | A collective term for Star Clusters, Nebulas, and Galaxies. It originally referred to objects that may be mistaken for a comet, thus any fuzzy object in the night sky. Famous lists of Deep-Sky Objects include the Messier, NGC-IC and PGC catalogs. The Caldwell and Levy List of Objects are also popular among amateur astronomers. |
| Diffuse Nebula | Irregularly shaped nebulae that resemble loose patches of clouds. Depending on their brightness these nebulae are classified as either bright nebulae or dark nebulae.
An example of a diffuse nebula is The Great Orion Nebula, classified as a bright nebula and is visible to the naked eye in the sword of Orion as a dim fuzzy star. |
| Disk | The circular shapes of spherical bodies like the sun, moon, and planets as seen on the celestial sphere. |
| Dobsonian Mount | A very cheap and effective method of mounting telescopes, invented by John Dobson, and now extremely popular, especially for cheaper telescopes. A Dobsonian mount consists of a flat "base plate" which sits on the ground and allows the telescope to rotate in azimuth, on top of which is a "rocker box" in which the telescope tube is pivoted, allowing the telescope to move in altitude. |
| Double Star | Stars that appear to be two stars when seen through optical instruments such as a pair of binoculars. Some double star are true binary star systems, but most are simply due to chance alignment with the earth. |
| Earthshine | Light illuminating the dark side of the moon during the crescent phases. It is really sunlight reflected from the earth's clouds and oceans. The earth itself does not emit any light of its own. |
| Eclipse | A phenomenon in which the light from a celestial body is temporarily cut off by the presence of another, either by passing through the shadow of the second body or being obscured by it. The second type of phenomenon is more strictly called an occultation.
The most spectacular eclipses are lunar eclipses and solar eclipses, although other eclipses are possible, such as eclipses of the satellites of Jupiter. |
| Eclipsing Binaries | Variable stars that are really binary or multiple star systems. Their total brightness varies on a regular cycle because their orbital position causes them to pass in front of one another as viewed from Earth. |
| Ecliptic | An imaginary great circle where the plane of the earth's orbit cuts through the celestial sphere. As the earth moves around the sun during the year, the sun seems to trace a circular path along the celestial sphere and back to where it started after exactly a year. This path is the ecliptic and is where the moon and most planets can approximately be found. |
| Ecliptic Poles | The two points where the projection of the perpendicular to the earth's orbit meet the celestial sphere. |
| Ecliptic System | A coordinate system on the celestial sphere using the ecliptic as its base circle. It is composed of the ecliptic latitude and ecliptic longitude coordinates.
Ecliptic latitude is the number of degrees an object is north or south of the ecliptic, while ecliptic longitude is the angle around a circle parallel to the ecliptic counting eastward from the Vernal Equinox. |
| Emission Nebula | A type of bright nebula that produces enough radiation to emit light of its own. This usually occurs when radiation from nearby stars is strong enough to start nuclear reactions in the gases of the nebula. Emission nebulae are the usual birthplaces of new stars.
The Great Orion Nebula at the sword of Orion is an example of an emission nebula with several young stars forming in its clouds. |
| Ephemeris | A table giving the equatorial coordinates, magnitude and other data for astronomical bodies such as the moon, sun, planets and comets.
Ephemeris tables and other information are often compiled in a book called an Almanac. An example of an almanac is the Philippine Astronomical Handbook published by PAGASA. |
| Equatorial Mount | A type of mount in which one of the telescope's axes is parallel with the Earth's axis of rotation. This allows the telescope to track the stars as the earth rotates simply by rotating around that one axis. |
| Equatorial System | A coordinate system on the celestial sphere using the celestial equator as its base circle. It is composed of the declination and right ascension coordinates.
Declination is the number of degrees an object is north or south of the celestial equator, while right ascension is the angle around a circle parallel to the celestial equator counting eastward from the Vernal Equinox. |
| Exit Pupil | The image that an eyepiece forms of the primary lens or mirror of a telescope. The diameter of the exit pupil can be computed by dividing the aperture of the telescope by its magnification. |
| Eyepiece | A combination of small lenses mounted in a tube, used to magnify and focus the image formed by a telescope or other optical instrument.
Eyepieces used for visual observation with a telescope are normally interchangeable, pushing or screwing into a draw-tube of standard size. |
| Field of View | The angular extent of the image formed by an optical instrument, such as a telescope. The field of view of a telescope can be found by dividing the apparent field by the magnification. |
| Filter | An accessory used with an optical instrument to narrow down the wavelength band passing into the instrument. Colour filters are used bring out particular features of planets. A very narrow-band filter may be used to isolate, for example, the hydrogen alpha spectral line, particularly for making observations of the Sun. |
| Finder | A small telescope attached to the tube of a larger instrument, used for locating the correct field of view. A finder usually incorporates a cross-wire and is aligned so that an object located on the cross-wire appears centrally in the smaller field of view of the main telescope. |
| Fireball | Any meteor with magnitude brighter than -4. Because this is approximately the magnitude of Venus, it is just any meteor brighter than Venus.
Any fireballs seen should immediately be reported to the Fireball Data Center of the International Meteor Organization. The report should indicate the direction in which it fell, duration, approximate brightness, and angular length. |
| Focal Length | The distance between a lens or mirror and the point at which it brings parallel light rays to a focus. |
| Galactic Center | The central region of the Milky Way which is used as the origin for galactic coordinates. |
| Galactic Circle | An imaginary great circle where the plane of the sun's orbit around the center of the Milky Way cuts through the celestial sphere. The galactic circle is where the greatest concentration of stars, star clusters, and nebulae can be found. |
| Galactic System | A coordinate system on the celestial sphere using the galactic circle as its base circle. It is composed of the galactic latitude and galactic longitude coordinates.
Galactic latitude is the number of degrees an object is north or south of the galactic circle, while galactic longitude is the angle around a circle parallel to the galactic circle counting eastward from the galactic center. |
| Galaxy | A massive ensemble of hundreds of millions of stars, all gravitationally interacting, and orbiting about a common center.
Astronomers estimate that there are about 125 billion galaxies in the universe. The sun and all the planets revolving around it belong to the Milky Way galaxy. |
| Gibbous | The phase of the moon or a planet when more than half of the disk, but not the entire disk, is illuminated. |
| Globular Cluster | A densely packed, roughly spherical star cluster of many thousands or even millions of stars. Globular clusters are much older and formed from original galactic material.
Although they have more stars, they generally appear as faint diffuse objects. The brightest globular cluster is omega Centauri, also the largest known cluster with a diameter of 620 light years. |
| Hydrogen Alpha | The most prominent line in the visible part of the hydrogen spectrum. It is the first line of the Balmer Series, with a wavelength of 656.28 nanometres, and is red in colour. It arises from transitions between the second and third energy levels in the hydrogen atom. |
| Inferior Planet | Any of the planets which are nearer to the sun than the earth, as opposed to the superior planets. Currently this refers to only Mercury and Venus. |
| International Astronomical Union | An organization formed in 1919 for international cooperation in astronomy. It is composed of member countries (represented by national academies or similar institutions, not governments) and around 8,000 individual members.
The IAU belongs to the International Council of Scientific Unions (ICSU) based in Paris. |
| Jovian Planet | A collective name for the planets Jupiter, Saturn, Uranus and Neptune, which share the general property of being gaseous giants in contrast with the rocky terrestrial planets. |
| Latitude | The number of degrees an object is north or south of the earth's equator. |
| Light Curve | A graph on which the magnitude of a variable star or other varying astronomical object is plotted against time. |
| Light Pollution | The scattering of light from man-made sources into the night sky. This increases the background brightness of the sky above its natural level and interferes with astronomical observations.
Light pollution is worst when close to major cities such as Metro Manila and Cebu, and has become a growing concern among observers in the Philippines. |
| Limiting Magnitude | The magnitude of the faintest object that can be detected visually at a given time, using either the naked eye or an optical instrument such as a telescope. |
| Longitude | The angle around a circle parallel to the earth's equator counting eastward from the prime meridian. |
| Luminosity | The energy radiated per unit time by a luminous body. |
| Lunar Eclipse | An eclipse of the moon, caused when the moon passes through the earth's shadow.
Lunar eclipses do not occur every month because the moon's orbit is slightly inclined from the earth's orbit. A lunar eclipse may be total, partial, or penumbral. |
| Lunar Grazing Occultation | A lunar occultation in which the occulted object appears just to touch the limb of the moon. |
| Lunar Occultation | The moon passing over a star, planet, or other celestial body. These phenomena are still important fields of observation because accurate timing of their occurances from several different observation points may improve the precision of a star's known coordinates.
The PAGASA conducts occultation timings at their observatory in UP Diliman, Quezon City. |
| Lunation | A complete cycle of the phases of the moon. The time taken for one lunation is called the synodic month. |
| Magnification | The factor by which the angular size of an object is apparently increased when it is imaged by a particular combination of a telescope and eyepiece. It is given by the focal length of the telescope divided by the focal length of the eyepiece. |
| Magnitude | A measure of the brightness of a star. Apparent magnitude is how bright a star appears to an observer on earth while Absolute magnitude is a measure of the actual brightness of a star.
Stars with the same absolute magnitude may have different apparent magnitudes, because those that are farther away appear dimmer than those that are much closer to the earth. |
| Maksutov-Cassegrain | A type of reflecting telescope which is similar to the Schmidt-Cassegrain but incorporating a deeply curved meniscus lens, which corrects the optical aberrations of the spherical primary mirror to give high-quality images over a wide field of view. The secondary mirror is fixed onto the back of the correcting lens and the image is formed just behind the primary mirror. |
| Manila Observatory | The first astronomical observatory and oldest institution of scientific research in the Philippines.
It once had the country's largest optical telescope—a 19-inch refractor along Padre Faura Street in Manila that was destroyed during the war. For more information, please visit the Manila Observatory website at www.observatory.ph. |
| Mare | A term used for the dark areas of the moon. Mare is a latin word meaning "sea", so called because they were once thought to be seas of liquid water. Today we know them to be seas of solidified lava. One of the most famous mare is Mare Tranquillitatis, because this is where the men first landed on the moon. The plural of mare is maria. Large maria are often called oceanus, latin for ocean. |
| Mercury Transit | Mercury crossing the solar disk, appearing as a tiny dot on the surface of the sun.
Mercury transits occur rarely because the orbits of Venus and Earth are not on the same plane. Venus transits can only happen in May or November, because these are where the planes of the two orbits intersect. A transit of Venus occurred in May 7, 2003 and will occur again on November 9, 2006. |
| Meteor Shower | Meteors observed to radiate from a single point in the sky, seen over a limited period, usually of several hours or days. The point at which meteors seem to radiate is called the radiant of the meteor shower.
Meteor showers occur annually. The Perseids occur during the rainy season and is therefore difficult to see. In the Philippines, the best shower to watch is the Geminids which falls around December 13. |
| Meteorite | The recovered fragment of a meteor that has survived passage through the Earth's atmosphere. The study and collection of meteorites is called meteoritics. Almost all meteorites come from debris originating from asteroids, because comet debris usually burns up completely while it is still high in the atmosphere. |
| Meteoroid | A small rock or dust particle in space with the potential to become a meteor. Most meteoroids are fragments of asteroids or comets that have broken up or collided with other objects.
Meteoroids that come from comets usually appear in the form of meteoroid streams along the same orbit as its parent comet, and become meteor showers if the earth crosses its path. |
| Meteoroid Stream | A stream of meteoroids orbiting the sun, usually along the orbit of a comet. When the earth crosses a meteoroid stream, we experience a meteor shower.
Meteoroid streams are caused by a comet ejecting material whenever it comes close to the sun. Thus, meteor showers are generally stronger if its parent comet has just passed perihelion. |
| Meteors | Also known as shooting stars or falling stars, are actually small rocks or dust particles from space that burn up upon entering the earth's atmosphere.
While in space, they are called meteoroids. When they reach the earth, they are called meteorites. Most meteors are sporadic, but occasionally several meteors that are part of the same meteoroid stream may form a meteor shower. |
| Milky Way | A large, disk-shaped galaxy, that includes the Sun and its solar system.
The Milky Way is our home galaxy. All stars visible in the night sky are part of the Milky Way galaxy, as it is not possible to see individual stars in other galaxies without the help of very powerful telescopes. |
| Mira Stars | A type of pulsating variable named after the group's prototype, Mira. |
| Monsoon | Strong prevailing winds over Southern Asia that changes direction with the change of seasons.
The Amihan (North-East monsoon) blows from November to May and generally brings good weather, while the Habagat (South-West monsoon) blows from May to November and is usually accompanied by heavy rains, called the monsoon rains. |
| Monsoon Rains | Heavy downpour that usually accompany the South-West Monsoon, or Habagat. This season is accompanied by skies that are unfavorable to astronomical observation. |
| Mount | The support for a telescope that allows it to be pointed at the desired position in the sky. Two main types are commonly used: the alt-azimuth mounting and the equatorial mounting. |
| Nebula | A localized mass of gas and fine dust particles in interstellar space. The term nebula (plural nebulae) is latin for cloud, and was formerly used for all fuzzy objects including galaxies and clusters, but is now used only for gas and dust clouds.
Seen from a telescope, they appear like patches of clouds in space. They are classified as diffuse nebulae, planetary nebulae, or supernova remnants. |
| Newtonian | A type of reflecting telescope with a parabolic primary mirror and a flat secondary mirror positioned at an angle of 45° which reflects the light onto an eyepiece at the side of the telescope's tube. |
| North Cardinal Point | A point along the astronomical horizon which is directly north of the observer. The North Cardinal Point, or simply the North point, is the foot of the perpendicular made from the North Celestial Pole to the astronomical horizon. It is directly opposite the South Cardinal Point. |
| North Celestial Pole | One of two points on the celestial sphere about which the sky appears to rotate daily. Their positions are where the earth's axis, if extended indefinitely, would intersect the celestial sphere. The North Celestial Pole is close the the star Polaris while the South Celestial Pole is close the the star sigma Octantis. |
| Nova | A star that suddenly increases in brightness by about ten magnitudes, then declines gradually over a period of months. The word nova is a shortening of the Latin phrase nova stella which means a new star. |
| Objective | The primary lens or mirror of a telescope, whose job is to focus parallel light onto a single point in order to produce an image of the object being observed. |
| Observatory | A building from which astronomers observe celestial objects. The main instrument in an astronomical observatory is usually a telescope.
The oldest observatory in the Philippines is the Manila Observatory founded in 1865. The official government observatory is the PAGASA Astronomical Observatory in UP Diliman, Quezon City. |
| Occultation | The passage of one astronomical object directly in front of another so as to obscure it from view as seen by a particular observer.
The most common type of occultations are lunar occultations, where the moon passes over a star, planet or other heavenly body. Other occultations are possible, such as Jupiter hiding one or more of its satellites. A solar eclipse is really an occultation. |
| Ocular Acuity | In astronomy, the ability to see very dim things more clearly. Observers sometimes look to the side of very dim objects instead of straight on to increase their ability to observe them. |
| Open Cluster | A spread out, loosely shaped star cluster of a few to several thousands of stars. Open clusters are relatively young and contain many hot, highly luminous stars.
On a telescope or binoculars, they appear as a group of bright, visually separate stars of about the same color. The most popular open cluster is the Pleiades, an open cluster with 7 bright stars visible to the naked eye. |
| Opposition | A planetary configuration wherein a superior planet is directly aligned with the earth and the sun. At opposition planets are at full phase and reach their highest point in the sky at midnight. At the same time, they achieve their closest approaches to the Earth. |
| Optical Telescope | A telescope that collects and focuses visible light from distant objects to produce a magnified image.
Optical telescopes are either refracting telescopes or reflecting telescopes, depending on whether they use a lens or a mirror respectively to focus light. The largest optical telescope in the Philippines is the PAGASA Astronomical Observatory Telescope found in UP Diliman, Quezon City. |
| Orrery | A working model of the solar system showing the planets, possibly with some of their moons, in their orbits around the Sun. The term "orrery" was first applied to such a model in 1713 when one was made for the Fourth Earl of Cork and Orrery. |
| PAGASA | Short for Philippine Atmospheric Geophysical and Astronomical Services Administration, the government body that studies Meteorology and Astronomy.
The PAGASA Observatory Telescope in UP Diliman, Quezon City is the largest optical telescope in the Philippines, with an aperture of 45 cm. For more information, please visit the PAGASA website at www.pagasa.dost.gov.ph. |
| PAS | The Philippine Astronomical Society, the first amateur astronomy organization in the Philippines.
PAS founded on April 24, 1971 by Phillip Wyman, Fr. Victor Badillo, Hans Arber, Hadrian Arroyo, Ernesto Damasco, Robert Kaul and Gertrudes Lucas. For more information, please visit the PAS website at www.philastrosoc.com. |
| Partial Lunar Eclipse | A type of lunar eclipse in which the moon only partly enters the earth's umbra, or darker shadow. |
| Partial Solar Eclipse | A solar eclipse which is neither total nor annular. In a partial solar eclipse, only a fraction of the sun's disk is obscured by the moon. A partial solar eclipse does not cause any noticeable darkening of the surroundings. |
| Penumbral Lunar Eclipse | A type of lunar eclipse in which the moon does not enter the earth's umbra, or darker shadow, but does enters the earth's penumbra, or lighter shadow.
A penumbral eclipse is hardly detectable, causing only a slight loss in the brightness of the full moon. |
| Perigee | The position in an orbit around the earth which an object is closest to the earth. It is also used to describe the time at which an object in earth orbit (such as the moon or an artificial satellite) reaches that position in its orbit.
Because objects orbiting the earth move in elliptical paths rather than perfect circles, all of them have both a perigee and an apogee. |
| Perihelion | The position in an orbit around the sun which an object is closest to the sun. It is also used to describe the time at which an object in solar orbit (such as a planet or a comet) reaches that position in its orbit.
Because objects orbiting the sun move in elliptical paths rather than perfect circles, all of them have both a perihelion and an aphelion. The earth is at perihelion at around January 4 each year. |
| Period | The amount of time taken for an object to complete an orbit, or the length of time between recurrences of a periodic phenomenom (e.g., between two successive maxima of a variable star). |
| Peripheral Vision | Not looking directly at an object. In observation, this sometimes allows you to see details not easily discernable when looking directly. |
| Phases of the Moon | Different parts of the lunar cycle marked by changes in the apparent shape of the moon. Phases are caused by the changing angle of sunlight hitting the moon.
New Moon, First Quarter, Full Moon and Last Quarter are officially defined as the times when the moon, the earth and the sun are at an angle of 0°, 90°, 180° and 270° with each other respectively. |
| Photometer | An instrument that can accurately measure the magnitudes of stars, or other astronomical objects, within specified wavelength bands. Photometric measurements can be used to deduce broad physical characteristics of stars, and are also important in determining the light curves of variable stars. |
| Photosphere | The visible surface of the Sun or a star. The photosphere is about 500 kilometres thick and is where the character of the gaseous layers changes from being completely opaque to radiation to being transparent.
The light which we actually see emitted from the sun comes from this layer. |
| Planet | Any of the nine major bodies orbiting the sun, namely, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
Pluto's status as a planet is questioned because of the presence of several bodies of similar nature located in a belt along its orbit. A tenth "planet" codenamed Xena was discovered to be larger than Pluto, but its status as a planet has yet to be confirmed by the IAU. |
| Planetarium | A dome-shaped building with a special projector used to simulate the appearance of the night sky. Planetariums are widely used for both educational purposes and entertainment.
Among the major planetariums in the Philippines are the Manila Planetarium in Rizal Park and the PAGASA Planetarium in Agham Road, Diliman, Quezon City. |
| Planetary Nebula | Spherical shaped nebulae so called because many of them superficially resemble planets through telescopes.
They are actually expanding shells of material that an old average star sheds during a late, red giant stage in its evolution, before becoming a white dwarf. An example of a planetary nebula is the Ring Nebula in constellation Lyra. |
| Precession | The slow rotation of the celestial poles in a 23.5° circle centred on the ecliptic poles, a complete rotation taking 25,760 years. The effect of precession is to slowly change the positions of the celestial poles against the stars, and also to slowly change the equatorial coordinates of a fixed object such as a star or galaxy. |
| Pulsating Variables | Variable stars that rhythmically brighten and fade due to changes in the stars' outer layers. Pulsating variables change in luminosity, temperature, and other characteristics as they expand and contract. These include Cepheid variables, RR Lyrae stars and Mira stars. |
| Quadrature | A configuration between two bodies on the celestial sphere that are at an angle of 90° from each other. This means that the two objects are perpendicular to each other as seen from earth. |
| RR Lyrae Stars | A type of pulsating variable named after the group's prototype, RR Lyrae. |
| Radiant | The point on the celestial sphere from which meteors belonging to a particular meteor shower appear to radiate.
Meteors entering the Earth's atmosphere from a stream move in paths that are almost parallel; the apparent divergence from a radiant is simply a perspective effect. |
| Radio Telescope | A telescope that collects and focuses radio waves from distant objects for later analysis. Radio waves have frequencies less than infrared light.
Astronomers use radio telescopes to study the sun, Jupiter, interstellar gas, and cosmic background radiation. Pulsars and quasars were discovered using radio telescopes. |
| Red Filter | Red cellophane or other translucent material used to cover flashlights during observation. This is essential for astronomers to keep the eyes dark adapted. |
| Reflecting Telescope | A kind of optical telescope that uses a concave mirror to collect and focus visible light to produce an image in the eyepiece.
There are several types of reflectors depending on the the way light is focused, the most popular being the Newtonian, Schmidt-Cassegrain, and Maksutov types of focus. |
| Reflection Nebula | A type of bright nebula that does not emit light of its own, but contains enough dust particles to reflect and scatter light from nearby stars. It has a characteristic blue color in contrast to the red glow of an emission nebula.
An example of a reflection nebula is the cloud surrounding the stars of the Pleiades star cluster in constellation Taurus. |
| Refracting Telescope | A kind of optical telescope that uses a convex lens to collect and focus visible light to produce an image in the eyepiece. |
| Satellite | Any body in orbit around a larger parent body. Most planets in the solar system have natural satellites, otherwise known as moons. Artificial satellites are man-made objects launched into orbit around the Earth, or another moon or planet. |
| Schmidt-Cassegrain | A type of reflecting telescope with a corrector plate to compensate for spherical abberation before a spherical primary mirror, and a convex secondary mirror which reflects the light back down the tube and through a hole in the primary mirror, onto an eyepiece at the back of the telescope's tube. The resulting telescope is compact, making it very portable. |
| Seeing | The effect of random motion in the atmosphere on the quality of the image of an object. In conditions of good seeing, images are sharp and steady; when seeing is poor, they are extended and blurred and appear to be in constant motion.
Amateur astronomers use the Antoniadi Scale to describe seeing, while professionals measure the number of arc seconds a point is displaced. |
| Simulacrum | Lunar or planetary features that appear to form recognizable objects such as a rabbit or a human face. |
| Solar Eclipse | An eclipse of the sun, caused when the moon passes between the earth and the sun. A solar eclipse is more strictly an occultation, but use of the term eclipse is deeply rooted in tradition.
Solar eclipses do not occur every month because the moon's orbit is slightly inclined from the earth's orbit. A solar eclipse may be total, annular, or partial. |
| Solar Filters | Filters designed to reduce the intensity of the sun's light and other radiant energy in solar observation, such as during solar eclipses and transits of Mercury or Venus.
Solar filters must allow at most one part in 100,000 to be safe enough to use. Neutral density 5 or 6 Mylar filter or welder's glass #13 or #14 are safe, but anything less could result in permanent blindness. |
| Solar System | The Sun, together with the planets and moons, comets, asteroids, meteoroid streams and interplanetary medium held captive by the Sun's gravitational attraction. The solar system is presumed to have formed from a rotating disc of gas and dust created around the Sun as it contracted to form a star, about five billion years ago. |
| South Cardinal Point | A point along the astronomical horizon which is directly south of the observer. The South Cardinal Point, or simply the South point, is the foot of the perpendicular made from the South Celestial Pole to the astronomical horizon. It is directly opposite the North Cardinal Point. |
| South Celestial Pole | One of two points on the celestial sphere about which the sky appears to rotate daily. Their positions are where the earth's axis, if extended indefinitely, would intersect the celestial sphere. The North Celestial Pole is close the the star Polaris while the South Celestial Pole is close the the star sigma Octantis. |
| Sporadic Meteors | Meteors that are not part of a meteor shower. Sporadic meteors may come from any random part of the sky and are not associated with any known stream of particles in space. |
| Star | An intrinsically luminous ball of gas generating energy in its hot core through nuclear fusion processes. They are seen as points of light by observers on the earth. |
| Star Cloud | An area of sky, particularly in the Milky Way, where large numbers of stars are seen close together, giving a cloud-like effect. |
| Star Cluster | Star systems that are gravitationally attached and travel together through space. Stars in a cluster have similar compositions and are believed to be of the same age and origin.
Star clusters may be made up of two or three stars or as many as several millions of stars. The two kinds of star clusters are open clusters and globular clusters. |
| Star Map | A map of the stars and constellations, often including deep-sky objects, double stars, and variable stars, and boundary lines between constellations.
The most famous star maps are Norton's 2000.0 Star Atlas and Wil Tirion's Sky Atlass 2000.0. Star maps for the Philippines include PAGASA's Philippine Star Maps and Bernie Esporlas's Starry Night. |
| Sunspot | A region on the Sun where the temperature is lower than its surroundings, thus appearing relatively dark. A strong magnetic field produces the cooling effect. Sunspots often form groups or pairs of opposite magnetic polarity.
The dark central part of the sunspot, the umbra, has a temperature of about 3,700 K compared with the 5,700 K of the photosphere. |
| Superior Planet | Any of the planets which are farther away from the sun than the earth, as opposed to the inferior planets. Currently these are Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. |
| Supernova | A violent explosion that occurs when a large star uses up its supply of fuel, collapses under its own weight, and explodes. Supernovas are rare phenomena—fewer than five supernovas in our Milky Way galaxy have been visible from Earth in the last 1,000 years. The most famous of these occurred in July 4, 1054. It was perhaps as bright as the full moon, and visible even in daylight for as long as three weeks. |
| Supernova Remnant | This type of nebula is really the remains of an exploding star. It is created by ejection of the outer layers of a star that explodes as a supernova.
An example of a supernova remnant is the Crab Nebula in Taurus, formed during the famous supernova of 1054. |
| Synodic Month | The average interval of time between two successive new moons (or any other specified phase), which is 29.53059 days. |
| Syzygy | A configuration between two bodies on the celestial sphere that at an angle of either 0° or 180° from each other. This means that the two objects are aligned with the earth, either on the same side or on opposite sides. |
| Telescope | An instrument that allows very distant objects to be observed by collecting large amounts of radiation. Astronomers use the telescope to study radiant sources that would otherwise be too faint to be detected.
The telescope usually means an optical telescope, but now includes devices that view other parts of the electromagnetic spectrum, such as x-ray telescopes and radio telescopes. |
| Terrestrial Planet | One of the inner rocky planets (Mercury, Venus, Earth and Mars), which are similar in fundamental structure to the Earth, as opposed to the jovian planets. |
| Total Lunar Eclipse | A type of lunar eclipse in which the moon fully enters the earth's umbra, or darker shadow.
In a total lunar eclipse, the moon does not really disappear, but only turns dark red. This is due to the refraction of light from the earth's atmosphere, allowing some sunlight to reach the moon. A total lunar eclipse occurred in May 5, 2004, and will occur again on March 4, 2007. |
| Total Solar Eclipse | A type of solar eclipse in which the sun is fully obscured by the moon. This occurs when the moon is near enough to the earth such that its disk appears larger than that of the sun.
In a total solar eclipse, the sky noticeably darkens and the horizon turns reddish as in dusk. The last total solar eclipse in the Philippines occurred in October 24, 1995 over Languyan island in Tawi-Tawi. |
| Totality | That period of time during a total solar eclipse when the sun is fully obscured by the moon.
The longest ever recorded duration of totality in the world occurred on June 20, 1955 in the Philippines, recorded at the PAGASA Astronomical Observatory in Quezon City. |
| Variable Stars | Star whose magnitude changes with time, whether regularly or irregularly. Variable stars are classified as eclipsing binaries, pulsating variables, or cataclysmic variables. |
| Venus Transit | Venus crossing the solar disk, appearing as a tiny dot on the surface of the sun.
Venus transits occur rarely because the orbits of Venus and Earth are not on the same plane. Venus transits can only happen in June or December, because these are where the planes of the two orbits intersect. A transit of Venus occurred in June 8, 2004 and will occur again on June 6, 2012. |
| Vernal Equinox | One of two points on the celestial sphere where the celestial equator and the ecliptic intersect. The Vernal Equinox is the point where the sun crosses the equator from south to north, while the Autumnal Equinox is where the sun crosses the equator from north to south. |
| Waning | The part of the lunar cycle when the illuminated portion of the moon's disc is decreasing. The opposite is "waxing". |
| Waxing | The part of the lunar cycle when the illuminated portion of the moon's disc is increasing. The opposite is "waning". |
| X-Ray Telescope | A telescope that collects and focuses X-rays from distant objects for later analysis. X-rays have frequencies greater than ultraviolet light.
All X-ray telescopes are mounted on rockets or satellites because radiation of these frequencies cannot penetrate the earth's atmosphere. The first X-ray source to be discovered was Scorpius X-1. |
