Astronomy

Celestial Equator

The extension of Earth’s equator onto the celestial sphere

Local Sky

The sky viewed from a particular location on Earth (or another solid object)

Angular Size

A measure of the angle formed by extending imagery lines outward from our eyes to span an object (or the space between two objects).

Circumpolar

A star that always remains above the horizon for a particular latitude

Stellar Parallax

The apparent shift in the position of a nearby star (relative to distant objects) that occurs as we view the star from different positions in Earth’s orbit of the Sun each year.

June Solstice

Both the point on the celestial sphere where the ecliptic is the farthest north of the celestial equator and the moment in time when the Sun appears at that point each year (around June 21st).

Horizon

A boundary that divides what we can see from what we cannot see

March Equinox

Both the point in Pisces on the celestial sphere where the ecliptic crosses the celestial equator and the moment in time when the Sun appears at that point each year (around March 21st).

Full Shadow

The dark, central region of a shadow

Synchronous Rotation

The rotation of an object that always shows the same face to an object that it is orbiting because its rotation period and orbital period are equal.

Sidereal Day

The time of 23 hours, 56 minutes, 4.09 seconds between successive appearances of any particular star on the meridian; essentially, the true rotation period of Earth.

Eclipse seasons

Periods during which lunar and solar eclipses can occur because the nodes of the Moon’s orbit are aligned with Earth and the Sun.

Solar Eclipse

An event that occurs when the Moon’s shadow falls on Earth, which can happen only at new moon. A solar eclipse may be total, partial, or annular.

Direction

One of the two coordinates (the other is altitude) needed to pinpoint an object in the local sky. It is the direction, such as north, south, east, or west, in which you must face to see the object. See also azimuth.

Latitude

The angular north-south distance between Earth’s equator and a location on Earth’s surface

Partial shadow

The lighter, outlying regions of a shadow.

Constellation

A region of the sky; 88 official constellations cover the celestial sphere.

Total lunar eclipse

A lunar eclipse in which the Moon becomes fully covered by Earth’s full shadow

Zenith

The point directly overhead, which has an altitude of 90 degrees.

Apparent retrograde motion

The apparent motion of a planet, as viewed from Earth, during the period of a few weeks or months when it moves westward relative to the stars in our sky.