General Astronomy, 29:61
Fall, 2005
September 18, 2005
Glossary: Positional Astronomy (Material of First Four Weeks)

Planetary Atmospheres I

• Pressure Scale Height: The height in the atmosphere for the atmospheric pressure to drop to 37% of its value at sea level. For the Earth, this is about 8.8 kilometers.

Overview of the Solar System

• Astronomical Unit: the mean distance between the Sun and Earth, $= 1.496 \times 10^8$ kilometers.
• Terrestrial Planets: Mercury, Venus, Earth, Mars
• Jovian Planets: Jupiter, Saturn, Uranus, Neptune
• Plane of the Ecliptic: the plane which contains the Earth's orbit around the Sun.
• Inclination: the angle between two intersecting planes. The first plane is the plane of the ecliptic. The second plane is the plane of the orbit of another solar system object.
• Satellites: moons of the planets
• Asteroids: small planets (maximum diameter of 1000 kilometers for Ceres), most of which are in the asteroid belt between Mars and Jupiter.

Astronomical Coordinate Systems

• Horizon Coordinate System: A coordinate system based on an observers horizon, four cardinal directions, and zenith.
• Azimuth: One of the two coordinates in the horizon coordinate system, an angle indicating the direction, starting a $0^{\circ}$ due north, moving to the east with an azimuth of $90^{\circ}$ due east, $180^{\circ}$ due south, etc.
• Altitude Angle: the second coordinate in the horizon system. The altitude says how high in the sky and object is seen. An object on the horizon has an altitude angle of $0^{\circ}$, one halfway up the sky has $Al=45^{\circ}$, and an object straight overhead at the zenith has $Al=90^{\circ}$.
• Zenith: the imaginary point in the sky straight overhead.
• Meridian: a (very important) imaginary line on the sky which goes from the horizon due north, through the zenith, and then down to the horizon due south. It divides the sky into the east and western halves.
• North Celestial Pole: the point on the sky corresponding to the intersection of the Earth's rotation axis with the celestial sphere. The South Celestial Pole is 180 degrees away.
• Celestial Equator: a line on the sky corresponding to the intersection of the plane containing the Earth's equator with the celestial sphere.
• The Ecliptic: a line on the sky corresponding to the intersection of the plane of ecliptic (see above) with the celestial sphere. The Sun, Moon, and planets are always seen very close (within a few degrees) of the ecliptic
• Celestial Coordinate System: A coordinate system fixed with respect to the stars. Every star has its two celestial coordinates, which do not change with time. The Sun, Moon, and planets also have celestial coordinates, but these coordinates change as the object moves around the sky.
• Right Ascension: one of the two coordinates in the celestial coordinate system. Right Ascension corresponds to longitude on Earth, and is measured in hours, minutes, and seconds. The origin of Right Ascension (where the value of this coordinate is zero) is at the point where the ecliptic intersects the celestial equator in the constellation of Pisces. This point on the sky is also called the vernal equinox or ``the first point of Aries''.
• Declination: the second of the two coordinates in the celestial coordinate system. Declination is the equivalent of latitude on Earth, and its units are degrees. An object on the celestial equator has a declination of 0, an object at the north celestial pole has a declination of $90^{\circ}$.
• Transit: an astronomical event corresponding to a celestial object moving through the meridian
• Obliquity of the Ecliptic: the angle between rotation axis of the Earth, and a line perpendicular to the plane of the ecliptic. The value of the obliquity of the ecliptic is $23^{\circ}.5$

Time and the Seasons

• Local Apparent Noon: the time of day when the Sun is on the meridian at a given place
• Apparent Solar Day: the interval of time between two successive transits of the Sun
• Mean Solar Day: the average of the apparent solar day over a year
• Apparent Solar Time: the amount of time before (for morning) or after (afternoon) local apparent noon
• Mean Solar Time: the apparent solar time which would occur if every day had the same length as the mean solar day. A simpler view would be to think of it as standard civil time, in which every day has the same length (mean solar day) and noon every day is at the time of mean noon
• Sidereal Day: the interval of time between successive transits of a star
• Sidereal Time: the Right Ascension of a star which is transiting right now
• Rotation: the motion of the Earth or another planet spinning on its axis
• Revolution: the motion of the Earth or another planet moving in its orbit about the Sun
• The Equation of Time: the difference between local apparent noon and mean local noon. The value of the equation of time can amount to as much as 16 minutes. A plot of the equation of time is referred to by the Greek word analemma.
• Summer Solstice: The term can refer either to an astronomical event, the time of year when the Sun reaches its highest declination, or a position on the sky, which is the point on the ecliptic which has the highest declination.
• Vernal Equinox: The term can refer either to an astronomical event, which is the time when the Sun moves across the celestial equator, or the point on the sky where the ecliptic crosses the celestial equator in the constellation of Pisces.
• Autumnal Equinox: The term can refer either to an astronomical event, which is the second time in the year when the Sun moves across the celestial equator, or the point on the sky where the ecliptic crosses the celestial equator in the constellation of Virgo.
• Winter Solstice: The term can refer either to an astronomical event, the time of year when the Sun reaches its lowest declination, or a position on the sky, which is the point on the ecliptic which has the lowest declination.
• Precession: The slow, periodic motion of the north celestial pole through an arc on the sky. The period of precession is 26,000 years.
• Solar Constant: the energy flux in the form of sunlight. The solar constant has the value of 1370 Watts per square meter. It is what would be measured by a perfect absorber one square meter in area, held perpendicular to the direction to the Sun, in space above the atmosphere of the Earth.

Calendars and Timekeeping

• Tropical Year: the interval of time between two successive passages of the vernal equinox by the Sun. The length of the tropical year is 365.2422 days.
• Sidereal Year: the interval of time for the Sun to return to the same place in the sky relative to the stars. The length of the sidereal year is 265.2564 days.
• Leap Year: the practice of inserting an extra day in the year every four years, so that the average length of the year is very close to the length of the tropical year.
• Sidereal Month: the interval of time for the Moon to go completely around the sky and return to the same place relative to the stars. The length of the sidereal month is 27.322 days.
• Synodic Month: the interval of time between reoccurrence of the same phase of the Moon. The length of the synodic month is 29.531 days.
• Perigee: the point on the orbit of any object orbiting the Earth (including the Moon) when it is closest to the Earth.
• Apogee: the point on the orbit of any object orbiting the Earth (including the Moon) when it is farthest from the Earth.
• Line of Nodes: the line corresponding to the intersection of an orbital plane (including that of the Moon) with the plane of the ecliptic.
• Solar Eclipse: an astronomical phenomenon which occurs when the Moon moves between the Sun and Earth, and blocks out the light of the Sun.
• Lunar Eclipse: an astronomical phenomenon which occurs when the Earth moves between the Sun and Moon, and casts a shadow on the Moon.