The celestial sphere
Long before telescopes, people mapped the sky with their eyes. Understanding how the sky appears to move is the first step in astronomy.
The celestial sphere
Astronomers imagine the sky as a giant celestial sphere surrounding Earth, with stars fixed on it. It is not real, but it is a useful model for describing positions. The points directly above Earth's poles are the north and south celestial poles; the celestial equator is the projection of Earth's equator onto the sky.
Daily motion
Because Earth rotates west to east once a day, the sky appears to turn east to west. Stars, the Sun, and the Moon rise in the east and set in the west. In the Northern Hemisphere, stars near the north celestial pole, close to Polaris (the North Star), circle it without setting; these are circumpolar stars.
Your position matters
The altitude of the celestial pole above your horizon equals your latitude. At the North Pole (latitude 90 degrees), Polaris is overhead; at the equator, it sits on the horizon. This is why different stars are visible from different places, and why navigators could find latitude from Polaris.
Constellations
Constellations are patterns of stars that cultures have named. The International Astronomical Union officially recognises 88 constellations, which divide the whole sky into regions, like countries on a map. Stars in a constellation usually lie at very different distances and are not physically connected; an asterism, such as the Big Dipper, is a smaller recognisable pattern.
Coordinates
Astronomers locate objects using right ascension and declination, sky equivalents of longitude and latitude. Declination is measured in degrees north or south of the celestial equator; right ascension is measured eastward in hours, minutes, and seconds. Angles on the sky are measured in degrees: the full Moon is about half a degree across.
The ecliptic and the zodiac
As Earth orbits the Sun, the Sun appears to move against the background stars along a path called the ecliptic, completing a circle in a year. The ecliptic passes through the zodiac constellations. Planets also stay near the ecliptic because the solar system is fairly flat.
Apparent motion of planets
Planets usually drift eastward relative to the stars, but sometimes appear to move backward (westward) for a few weeks: retrograde motion. This happens when Earth overtakes a slower outer planet, like a faster car passing a slower one, which seems to move backward against distant scenery.
Worked example
An observer in Madrid at latitude 40 degrees north sees Polaris 40 degrees above the northern horizon. Stars within 40 degrees of the north celestial pole are circumpolar there and never set. An observer in Singapore, near latitude 1 degree, sees Polaris barely above the horizon and has almost no circumpolar stars.
Key takeaways
The celestial sphere is a model with celestial poles and equator. Earth's rotation makes the sky turn east to west daily; circumpolar stars never set. The celestial pole's altitude equals your latitude. The IAU recognises 88 constellations; their stars are usually unrelated in distance. Right ascension and declination locate objects. The Sun moves along the ecliptic yearly. Retrograde motion occurs when Earth overtakes an outer planet.
Flashcard
Imaginary sphere of the sky
Check your understanding
Why does Mars sometimes appear to move backward against the stars?