Why the sky changes

Through the night: Earth spins

As Earth turns, the whole sky wheels around the celestial poles.

As Earth rotates, the stars appear to circle (opens in a new tab) the two points in the sky above its poles. Stars near a pole make small circles; stars farther away make large ones, rising and setting like the Sun.

That is why Polaris, close to the northern pole, stays in essentially the same spot (opens in a new tab) all night while the constellations swing around it.

Sources for this section Preston Dyches, What is the North Star and How Do You Find It?, NASA Science (2021, updated 2024) (opens in a new tab) · ; NASA Science, Skywatching FAQ (2024) (opens in a new tab)

Through the year: Earth orbits

Each night the stars rise a little earlier, so the evening sky slowly changes over the year.

NASA Space Place (opens in a new tab) explains that as Earth orbits the Sun our view into space at night changes, so the stars appear each night slightly west of where they were the night before.

Constellations behind the Sun are hidden in daylight. NASA's example: from the Northern Hemisphere on 21 September you can probably see Pisces, but not Virgo (opens in a new tab), which is then on the other side of the Sun.

Keep in mind There is no single best month for a constellation everywhere. This site gives the month when each is highest at about 9 p.m. local time, which holds in both hemispheres, calculated from the catalogue (opens in a new tab); the sky finder does it for your own place and night.

Sources for this section NASA Space Place, What Are Constellations? (updated 2022) (opens in a new tab) · ; Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), as computed by this site's star engine (opens in a new tab)

Where you are: latitude

Your latitude decides which stars never set, which rise and set, and which you can never see.

Your location on Earth (opens in a new tab) decides which stars and constellations you see and how high they rise; the Northern and Southern Hemispheres look out in different directions and see partly different skies.

Polaris's height above the horizon roughly equals your latitude: overhead at the North Pole (opens in a new tab), lower farther south, and gone south of the equator. Near the pole some stars never set; on the far side some never rise.

By this site's calculation (opens in a new tab), the whole Southern Cross is above the horizon only south of about 27°N, and Canopus (opens in a new tab), the second-brightest star, never rises north of about 37°N.

How to see it On a clear evening, note where the Big Dipper or Southern Cross is, then look again two hours later: it will have swung around the pole, as NASA describes (opens in a new tab).

Sources for this section NASA Space Place, What Are Constellations? (updated 2022) (opens in a new tab) · ; NASA Science, Skywatching FAQ (2024) (opens in a new tab) · ; Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), as computed by this site's star engine (opens in a new tab) · ; Jim Kaler, Canopus, STARS (University of Illinois) (opens in a new tab) · ; Preston Dyches, What is the North Star and How Do You Find It?, NASA Science (2021, updated 2024) (opens in a new tab)