Celestial Sphere

Zenith

Zenith

point on the celestial sphere that is directly over our heads - 90˚ above the horizon.

Nadir

a point directly opposite from zenith - 90˚ below the horizon.

Celestial Sphere

An imaginary sphere surrounding the Earth used by Astronomers to describe the location and movement of celestial objects.

Vernal Equinox

March 21 (in the northern hemisphere). Day and night are both 12h long.

North Celestial Pole

An extension of the Earth's north pole projected upon celestial sphere. All constellations appear to rotate around this point in the northern hemisphere.

South Celestial Pole

An extension of the Earth's south pole projected upon celestial sphere. All constellations appear to rotate around this point in the southern hemisphere.

Celestial Equator

An extension of the Earth's equator projected upon celestial sphere.

The Ecliptic

Plane in which the Earth moves around the Sun. Also the Sun's apparent annual path around the celestial sphere.

Earth's tilt

23degrees, 27minutes('). Responsible for the seasons.

Summer Solstice

June 21, the longest day of the year (in the northern hemisphere)

Winter Solstice

December 21, the shortest day of the year (in the northern hemisphere)

Azimuth

The angle that defines the horizontal location of a star or point in the sky. Usually measured clockwise starting from North, e.g. 45˚ is NE, 90˚ is East, etc.

Altitude

Angular elevation of a star above the horizon measured in degrees, minutes, and seconds.

Right Ascension

Longitude on the celestial sphere. Typically measured in hours and minutes instead of degrees. Tells you how long until a celestial object crosses your meridian (after some corrections).

Declination

The latitude of the celestial sphere. Measured in degrees, minutes, and seconds.

Constellation

A region of the sky that contains a recognizable star pattern and that is used to describe the location of objects in space. There are 88 in total.

Circumpolar Star

Above the horizon at all times for a given latitude. Appears to go in a circle around a Celestial Pole.

Horoscope

A forecast of a person's future, typically including a delineation of character and circumstances, based on the relative positions of the stars and planets at the time of that person's birth.

Meridian

Circle that goes through the north pole, zenith, south pole, and nadir. The best time to observe a star is as it crosses this line.

Autumn Equinox

September 21 (in the northern hemisphere). Day and night are both 12h long.

Astrology

The study of the movements and relative positions of celestial bodies interpreted as having an influence on human affairs and the natural world.

Horizon

Altitude of 0 degrees. "The line where the sky meets the sea" - Moana. You can only see above this line.

Angular Size

The angle an object take up in your field of view. Measured in degrees, minutes, and seconds. Depends on the linear size of the object and the distance to that object.

Linear Size

The physical size of an object measured in meters, or, for the truest of Americans, football fields .

Precession

The movement of the axis of rotation of a spinning object, e.g. the wobbling of a spinning top. Over the last 20,000 this has slowly "moved" the constellations. Don't tell the astrologers about it. They get very upset.