LCAS Dictionary of Useful Astronomical Terms

Abbreviations! - Almost every field of endeavor adopts a sort of shorthand - abbreviations that stand for frequently used words. We do the same. The article contains some abbreviations that have been used in the LCAS NightTimes.

Airglow - Even at dark sites in the desert, the night sky is not as black as you might expect. That's because of the naturally occurring airglow that results when elements in our atmosphere continue to interact with charged particles that had emanated from the sun during daylight hours. Atmospheric moisture in more humid parts of the country filters out this faint airglow, making the night sky appear darker, but also blocking some light from deep sky objects.

Albedo - Sunlight that falls on a body in the solar system is partially absorbed and partially reflected. The reflected fraction is known as the albedo and is expressed as a decimal number. A high albedo doesn't mean an object appears bright in our sky. For example, the albedo of Mars is .15, yet despite its brightness, the albedo of the moon is only .07. Stated simply, that means the surface of the moon is inherently darker than that of Mars.

Apastron and Periastron; Apogee and Perigee; Aphelion and Perihelion - If you recall the meanings of apogee and perigee, then you have a hint about apastron and periastron. These terms apply to the situation where one star revolves around another in an eliptical orbit. (virtually all orbits are eliptical.) When the secondary (smaller) star is closest to the primary, it's referred-to as being in periastron. At its farthest point, it's at apastron.

Astronomical Unit - One AU, or astronomical unit, is the distance from the Sun to the Earth - about 93 million miles or 148 million kilometers. It is used as a convenient unit of measure within the Solar System. When we say, for example, that Saturn is 9.5 AU from the Sun, we can see right away that it is about ten times farther out than we are. Saying that Saturn is about 880,000,000 miles out carries no such obvious information and requires us to absorb too many digits.

Aposelene and Periselene - Knowing that "selene" refers to our moon (as in "selenography"), you can probably figure out what these words mean. These terms are most commonly used in matters related to spaceflight. The periselene point in an elliptical orbit is the point of closest approach to the moon, while aposelene is the farthest point in the orbit. When a spacecraft arrives at the moon, it usually assumes an elliptical capture orbit. If the mission involves mapping, as was the case with the Lunar Prospector and Clementine spacecraft, then a later maneuver circularizes the orbit.

Argument of Perihelion - This is a measurement used in defining the shape of an object's orbit. It's used in combination with other numbers, called orbital elements, which provide a complete definition of the orbit. The symbol used to represent Argument of Perihelion is usually the lower case Greek letter omega (ω).

Astigmatism - Astigmatism in optics is a defect in the surface of a lens or mirror where the focus of a light beam varies depending on how far off center the light beam hits the lens or mirror.

Averted Vision - The optic nerve in your eye is positioned just about directly behind the pupil. When you look directly at an object, the image is focused on the inner part of the eye in the area of the nerve. Since there are fewer rods and cones in this spot, your direct vision of an object is less acute than your peripheral vision, where the object is focused in another part of the eye. Thus, when observing a faint object, you can see more detail if you focus your gaze just to one side of where the object lies. This is called averted vision.

Aspheric - This is an optical term that refers to the mirror of a reflecting telescope that is ground to the figure of a parabola or ellipsoid. Such a mirror is said to be "aspheric" - literally, "non-spheric".

BL Lac Objects - These are compact quasars with rapid, irregular changes in brightness of up to three magnitudes. They take their name from BL Lacertae, an object that was once thought to be a variable star but later found to be a quasar.

Blue Moon - People first used the words blue moon back in the year 1883, when the Krakatoa volcano erupted. It put a lot of dust into the air and the moon actually appeared to be blue through the dust. It was something you could see everywhere on earth and people talked about. These days the term blue moon does not refer to the color of the moon. Instead it means that a full moon occurs twice in the same month and is a rarity.

Cataclysmic Variable - The U Gem type of variable star is also referred-to as a dwarf nova, or a cataclysmic variable. These are binary stars, one a white dwarf, the other a subgiant star of spectral type K or M (cooler than the Sun, and larger in diameter). They are close enough that tides from the white dwarf rip gas from the bigger star; this matter spirals in on the white dwarf, forming an accretion disk. Usually the stars orbit one another every 1 to 12 hours.

Central Meridian - The central meridian of a planet tells you what part of that planet is directly facing Earth. For example, the feature Syrtis Major on Mars is located near Martian longitude 300o; it is therefore best observed when this longitude is near the Martian central meridian.

Colongitude - Colongitude is the longitude of the sunrise terminator on the moon. It's measured eastward from the mean center of the Earth-facing side of the lunar disk. Thus the colongitude is 0o at first quarter phase.

Coordinated Universal Time - By international agreement, this is the local time at the prime meridian, which passes through Greenwich, England.

Culmination - The point at which an object is exactly due south is also when it's at the highest point it can reach in the sky for your latitude. That is when the object is said to culminate.

Diffraction Limited - The term "diffraction limited" originated in 19th century England with Lord Rayleigh, who stated that a telescope with a wavefront error of 1/4 wave length of light would provide most of the performance that is possible when compared to a theoretically perfect optical system. That diffraction limited standard originally applied to unobstructed telescopes (refractors). In multi-mirror designs such as Newtonians and SCTs, wavefront errors in each optical element are cumulative in the telescope; thus better individual optics are needed in order to produce an overall diffraction limited system.

Eccentricity - Most objects that revolve around the Sun have orbits only slightly distorted from a circle, thus they're always at roughly the same distance from the Sun. Eccentricity is the measurement of this departure from a circular orbit. An eccentricity of 0 means the orbit is a circle. An eccentricity close to 1 means the orbit is very elliptical, as is typical of comets.

Elongation - This is generally used in relation to the planets Mercury and Venus, which never rise very high because their orbits lie between us and the Sun. Greatest elongation is when they seem to be at the farthest point east or west of the Sun; that is, when they appear highest in our sky.

Escape Velocity - If an object leaves Earth's atmosphere at about 11 km\/sec (7 miles\/second, or about 25,000 miles\/hour), it will escape the earth's gravity. This speed is called the escape velocity of the Earth. Below this speed, it will eventually come back to Earth. Other celestial bodies have different escape velocities.

Eye Relief - This term is a measure of distance to the point behind the eyepiece where you must position your eye so you can see the entire field of view.

Exoplanetisms - The search for planets around other stars has spawned new observing methods, and there have also come a few new terms. First of all, a planet circling another star is an exoplanet, meaning it's a planet in some other solar system.',CAPTION,'Exoplanetisms');

First Light - This term refers to the first time that a new instrument (generally an optical telescope) is used to actually observe an object in space. When a telescope begins its inaugural observing run, it's said to "see first light".

Geomagnetic Field - This refers to the magnetic field observed in and around the Earth. The intensity of the magnetic field at the Earth's surface is approximately 0.32 gauss at the equator and 0.62 gauss at the north pole.

Geosynchronous and Geostationary - The term geosynchronous applies to any satellite with an orbital velocity equal to the rotational velocity of the Earth. The net effect is that the satellite appears virtually motionless with respect to an observer on the ground. And another term: If a satellite's distance from the surface of the Earth remains constant, regardless of its orbit, it is said to be geostationary.

Ground Level Event - A ground level event (GLE) occurs when protons from the sun reach the Earth, penetrating the magnetosphere and atmosphere with sufficient energy to be detectable by cloud chambers and other neutron monitoring devices. These events are of special interest to the International Space Station.

Hilda Effect - The gravity of Jupiter has a strong effect on the orbits of asteroids, controlling some periods of revolution around the Sun. In particular, if an asteroid takes ⅔ as long as Jupiter to orbit the Sun, Jupiter's gravity will tend to hold it in that orbit. This is an example of resonance. The first asteroid found with this property happened to be named Hilda. Subsequent examples of this effect are known as members of the Hilda orbital family.

Incipient Resolution - If you have ever looked at a globular cluster that is not resolved into individual stars, but has a mottled appearance, you have experienced incipient resolution.

Ionization - An atom is in a neutral state when the electron and proton are associated. When the electron and proton are separated, the atom is said to be ionized.

Julian Day - The usual method of expressing dates in days, months, years, hours, minutes, and seconds can be tricky to deal with mathematically. If you try to figure out the difference in days between July 4, 1776 and today's date, the problem becomes evident. To avoid this, astronomers often express time in terms of Julian Day when making calculations, with decimal fractions corresponding to fractions of a day. JD 0.0 corresponds to January 1, 4713 BC; any other JD is the number of days since then. Thus, January 1, 2000 was JD 2451545.

Kidney Bean Effect - A very long focal length eyepiece used on a Newtonian telescope may exhibit the "kidney bean effect" - a black spot that seems to float in the center of the field of view. This is the shadow of your secondary mirror.

Kuiper Belt - In 1951, the astronomer Gerard Kuiper suggested that there might be billions of small, icy objects just beyond the orbits of Neptune and Pluto. These objects would be spread out over so large a volume that they wouldn't tend to collect to form actual planets, but would give a possible explanation as to why there are so many short-period comets. Since they tend to be of about magnitude 23, these objects were too dim to be detected until 1992. Since then, over 1200 Kuiper belt, or 'transneptunian objects' (TNOs) have been found.

Limiting Magnitude - Limiting magnitude indicates the faintest star a certain sized instrument will show, as indicated in the table in the article.

Light Bridge - When you observe a large sunspot system, you may notice light streaks that appear to cross the umbra, which is the darkest part of the sunspot. This is a light bridge. It might look like a solar flare, but flares are short lived and appear more as long-lasting "flashes".

Local Group - The Local Group consists of a number of galaxies that lie in the vicinity of the Milky Way and are all gravitationally bound to each other.

Mascon - This literally refers to a mass concentration - an area of the lunar crust characterized by an excess of mass. Those detected to date coincide with the circular maria, indicating the presence of relatively dense materials (basaltic lava) at a shallow depth.

Meteoroid - This is one of the most commonly misused terms. A Meteoroid is one of the countless small solid bodies in space, some no larger than a grain of sand. When it enters the Earth's atmosphere and ignites due to air friction, we see it as a streak of light that is then known as a Meteor. If it does not disintegrate entirely and lands on the Earth, only then is it referred to as a Meteorite. Sizes range from submicroscopic particles to bodies approaching the size of a small asteroid (also known as a planetesimal). Composition ranges from silicates to nickel-iron metal.

Metonic Cycle - The Metonic Cycle is a period of 19 years, or 235 lunations, after which time the phases of the Moon are repeated on the same days of the year. Thus (almost exactly) 19 years from today's date the moon will be at exactly the same phase as it is today. A lunation is the period of 29 days, 12 hours, 44 minutes from one new moon to the next, also known as a synodic month.

Morphology - Morphology isn't strictly an astronomical term; it refers to the form and structure of an object. Typically, astronomers refer to the "morphology" of a galaxy, which encompasses such factors as whether it's a spiral or elliptical, the prevalence of gas, and whether it's still in a star-forming stage.

Nanometer - A nanometer is one billionth of a meter (.000000001) and is a unit of measure for the wavelengths of different colors of light. Light also is measured in Angstroms (which are one-tenth of a nanometer), though the nanometer has recently come into widespread use. As an example, visible light has wavelengths between 400 and 700 nanometers.

Nebula Night - Once in awhile, observers will note that "It's a good night for nebulas". Obviously, we wouldn't be out observing deep sky objects unless the sky was clear and dark, but some nights just seem to particularly favor nebulae more than other types of deep sky objects.

Newton's Rings - Newton's rings are seen whenever two more or less smooth surfaces are close to touching each other, but are not exactly parallel. They're most commonly seen in oil slicks on wet pavement, where you see a rainbow of colors because the light is reflecting back and forth between the two surfaces (air-oil and oil-water). The colors are caused by differences in the thickness of the film.

Noctilucent Clouds - Noctilucent ('night-shining') clouds are thin, wispy clouds, glowing electric blue, that are usually seen after the sun has set. They occur on the fringes of space, at 80 to 100 km in the Earth's atmosphere. They were first reported in 1885 after the eruption of the volcano Krakatoa in Indonesia. While they have been regularly seen since then, their cause in uncertain. Some scientists think they're seeded by space dust, while others suggest they could be a telltale sign of global warming.

Parallax - Parallax is a method used to determine a star's distance. As the Earth orbits the Sun, our motion makes nearby stars appear to wobble back and forth as we see the stars first from one point in our orbit, then from another. This motion is always very small, but with good instruments, it can be measured.

Parfocal Eyepieces - You'll often see a series of eyepieces advertised as being "parfocal". This simply means that when you switch to a different eyepiece in the set, you can take out one eyepiece and insert another without having to refocus.

Phase Effect - As sunlight strikes an object in the solar system, the light is not reflected uniformly in all directions. The brightness of the reflection from a planet or satellite depends on the angle between the direction from the sun to the object and the direction of the Earth to the object. This dependence on the angle of incidence is known as the phase effect.',CAPTION,'Phase Effect');

Photographic Magnitude - The magnitude of a celestial object as measured on a photograph may be considerably different than that measured by the eye. This happens because most film emulsions react more strongly to red light than does the human eye, which tends to be more sensitive to blue light. An object that is redder in color will tend to have a brighter photographic magnitude than visual magnitude.

Position Angle - In order to describe the orientation of a galaxy, we can refer to its position angle. This angle is set up by drawing a line north from the center of the galaxy and a second line through the major ("long") axis of the galaxy.

Prominences - Prominences are H-Alpha emission features projecting beyond the limb of the sun, consisting of complex clouds or streamers of gas above or within the chromosphere. They generally come in two broad classes: Active (limb flares, surges, sprays, loops), and Quiescent (quiet region filaments, active region filaments). Filaments are merely prominences seen in absorption against the disk of the sun, so the terms have been used somewhat interchangeably.

Purkinje Effect - The Purkinje Effect is a unique result of how the human eye functions. For astronomical observers, this causes a reddish-colored star to appear brighter the longer it is looked at directly - sort of how film accumulates light the longer it is exposed.

Quadrature - Quadrature is when the direction of a heavenly body as seen from the Earth makes a right angle with the direction of the Sun. A superior planet (outside the Earth's orbit) is at west quadrature when its position is 90o west of the Sun. It rises around midnight, reaches the meridian near sunrise and sets near noon. At east quadrature the planet is near the meridian at sunset and sets near midnight.

Radiant - The radiant of a meteor shower is the point in the sky from which meteors appear to originate. If you were to draw lines back along the meteor trails, they would point to this area. The names of annual meteor showers come from the constellation where the radiant lies. When viewing a meteor shower, you don’t have to face the direction of the radiant; most of the meteors will not be seen near the radiant, but elsewhere in the sky.

Red or White Light Images - This refers to the fact that a colored filter was used to accentuate certain features.

Regolith - The term "soil" implies the presence of decomposed vegetable matter. But on places like the moon or Mars where the fine surface material is devoid of life, it's referred to as "regolith". As far as we know, true "soil" is unique to planet Earth.

Resolving Power - With larger aperture, a telescope mirror or lens is capable of showing fainter objects due to the increased light grasp. In addition, it's also capable of showing more minute detail. The ability to see fine detail is referred-to as resolving power, or simply resolution.

Retrograde Motion - If you were to plot or photograph one of the outer planets for a few months, you would note that its movement against the background stars is not always in the same direction. This apparent reversal of direction is referred to as retrograde motion.

Revolution and Rotation - A planet rotates around its axis, but revolves around the Sun.

Shepherd Moons - Very small satellites whose gravity helps keep the particles in a planet's ring system from dispersing are referred-to as "shepherd moons".

Spherical Aberration - On a lens or mirror with spherical aberration, light rays focus at a different point depending on what side of the optic the light hits. As an example, the left side may focus at 60 inches while the right focuses at 60.5. This uneven focus is across the entire surface of the optic, as opposed to astigmatism being from the center outward.

Sublimation - Sublimation refers to the situation where a substance goes from one state to another without passing through the normal intermediate state. A typical example: As a comet approaches the sun, the ice in the coma is heated, but because space is cold and airless, the ice goes directly into a gaseous state, rather than becoming a liquid (i.e.: turning into water).

Synodic Period - This refers to the time between two successive conjunctions of a planet with the sun, as viewed from the Earth. For the interior planets, an inferior conjunction is when they lie between us and the sun. A superior conjunction occurs for all the planets; it's when they pass behind the sun as seen from Earth.

"Theory" Defined - To be a scientific theory, a proposal must do two things: (1) Describe an aspect of reality/nature, and (2) be testable.

Turned-Edge - This means the outer edge of a lens or mirror is ground down too far or not ground enough. If the defect is not too great then it can sometimes be masked but this takes away from the total size of the lens or mirror.

Twilight - Twilight is defined in three ways: Civil twilight occurs when the sun is six degrees below the horizon. At the end of civil twilight, you're legally required to turn on your automobile headlights. The next darker form of twilight is nautical twilight, with the sun 12 degrees below the horizon. During nautical twilight, you can see stars and planets, but it is still bright enough for the horizon to be seen. Astronomical twilight occurs when the sun is 18 degrees below the horizon. At the end of astronomical twilight, the sky has reached its maximum darkness. In polar latitudes, nights may pass where one or more of these may not occur.

UT1 - UT1 is a measure of time based on the rotation angle of the Earth as observed astronomically. It is affected by small variations in the rotation of the Earth, and can differ slightly from civil time on the Greenwich meridian.

Virgo Infall - In theory, the expansion of the universe should be a pretty smooth process; if one galaxy is twice as far from us as another, it should appear to recede from us twice as rapidly. In reality, many factors conspire to make this only approximately correct. One reason this happens has to do with the nearby cluster of galaxies in Virgo. Its gravitational pull modifies the velocity of the Milky Way as it travels through the cosmos (and the velocity of some other galaxies, as well). If you compute that effect and subtract it out, then you get galaxy velocities that better match the Big Bang theory. At least one source, the Principal Galaxy Catalog (PGC), often lists such velocities corrected for this Virgo infall.

Zodiacal Light? Gegenschein? - When sunlight is reflected off these dust particles in our solar system, we see the zodiacal light, which is an elliptical glow along the ecliptic extending up from the horizon just after sunset or before sunrise.'

Zonal Defect - This is an optical defect that is isolated to a small part of a lens or mirror. Let's take an example: looking at a lens as if it were a clock face, it might only affect a small area between 1 and 2 o'clock, a quarter of the way in from the edge of the lens. This can be an area that is overcorrected or undercorrected but it does not occur all the way around the lens. A shorthand term often used is that the optic has "zones".

Zone of Avoidance - This is the area of the Milky Way Galaxy that contains such large amounts of dust that we cannot see what lies beyond it. The dust that blocks our view lies primarily toward the center of our galaxy in the direction of the constellation Sagittarius.