How the Night Sky Moves Back | Up | Next

The Earth's rotation makes the Sun, Moon, planets and stars seem to move across the sky.*

You may have noticed that the Sun rises in the morning in the east, moves across the sky during the day, and sets in the evening in the west. Have you ever wondered why?

It's because the Earth rotates on its axis, spinning like a top. It makes one rotation in 24 hours. This period defines the length of a day. The Sun is not really moving across the sky, rather, the Earth is turning.

The stars appear to move across the sky at what is called the sidereal rate. A telescope must track the stars at the sidereal rate to follow them as they move across the sky.

The Earth's axis of spin runs through the Earth through the north and south poles. If we extend this line out into the sky in space, we call these points the north and south celestial poles. The north celestial pole is conveniently located close to the fairly bright star Polaris. The southern hemisphere doesn't have any bright stars near the south celestial pole.

Stars leave trails in the sky that arc around the north celestial pole in this time-exposure photograph.

If we watch the night sky over time, the stars will all seem to revolve around the celestial pole. As the Earth goes around its axis of spin, the sky goes around the extension of this axis in space, the celestial pole. If you take a long time exposure pointed at the north or south celestial pole, all of the stars will make arcs around this point.

Astrophotography Considerations

The fact that the Earth rotates on its own axis also makes the Moon, planets and stars appear to move across the sky during the day and night as well. This is a very important consideration for taking astrophotos. For long time exposures these objects will not stay still fixed in a camera's field of view on a fixed tripod. They will move because of the Earth's rotation.

Stars trail in this 5-minute exposure of Orion taken with a DSLR camera on a fixed tripod and a wide-angle lens with 24mm of focal length.

How the Night Sky Moves - The Bottom Line

Because the Earth rotates on its own axis, celestial bodies seem to move across the sky.

For short exposures of less than about 30 seconds with wide-angle lenses, this movement will not be too noticeable.

But for longer exposures or with longer focal lengths, we must use a special mount for our camera or telescope to compensate for the Earth's rotation.

* Earth image by Reto Stockli, Alan Nelson, Fritz Hasler NASA Visible Earth. Milky Way image by Jerry Lodriguss. Solar Corona by Andreas Gada, composite and image processing by Jerry Lodriguss. Illustration composite and design by Jerry Lodriguss.

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