#### Definitions of Astronomical Terms

aphelion
The farthest point from the sun in a body's orbit
apogee
The farthest away the moon gets from the Earth
ascending node
The point in a body's orbit when it crosses the plane of the solar system while moving up
A.U. or Astronomical Unit
A measurement of distance equal to 149.6 Gm (149.6 million kilometers or 92 957 130 miles. It was originally defined as the average distance between the Earth and the Sun
descending node
The point in a body's orbit when it crosses the plane of the solar system while moving down
ecliptic
The imaginary line that traces the path of the sun through the sky. This line is actually the plane of Earth's orbit. All the planets follow this plane fairly closely except for Pluto.
greatest eastern elongation
When a planet inside our orbit (Mercury or Venus) reaches the point in it's orbit as far east as possible from the Earth's point of view
greatest western elongation
When a planet inside our orbit (Mercury or Venus) reaches the point in it's orbit as far west as possible from the Earth's point of view
inferior conjunction
When a planet is between the Earth and the Sun and is lost in the Sun's glare
light-year
A measure of distance equal to 9.46 Pm (9.46 trillion kilometers or 5.878 trillion miles) defined as the distance light travels in one year
magnitude
A logarithmic scale for measuring the brightness of a star the smaller the number, the brighter the star. Magnitude 6 is the dimmest star that can be seen by the naked eye. Each step in magnitude equals a difference in brightness of 2.512 times
occult
One body moves in front of and blocks the other
opposition
Two bodies are exactly opposite each other on the plane of the solar system
perigee
The point in the moon's orbit where it is closest to the Earth
perihelion
The point in a body's orbit where it is closest to the sun