## The

Earth's rotation provides a basis for keeping time using astronomical observations. The solar day of everyday affairs measures the time interval of Earth's rotation using the Sun for reference. The sidereal day measures the time interval of Earth's rotation using the stars for reference.

A sidereal day is 23 hours, 56 minutes, 4 seconds long. It is the time interval required for a star to cross your meridian two times successively, or the time for Earth to complete one whole turn in space. A solar day is 24 hours long, the length of time required for two successive meridian transits by the Sun.

A solar day is about 4 minutes longer than a sidereal day because while Earth rotates on its axis it also moves along in its orbit around the Sun. Earth must complete slightly more than one whole turn in space before the Sun reappears on your meridian (Figure 1.13).

A clock that keeps sidereal time is useful for stargazing. In sidereal time, all stars return to their identical positions in the sky every 24 hours. So a star rises, transits the meridian, and sets at the same sidereal time all year long.

You can use celestial coordinates (see Table 1.1) to determine the sidereal time at any instant when you are stargazing. Local sidereal time is equal to the right ascension of stars on your meridian. For example, if you see brilliant Sirius transit, the sidereal time is 6 hours, 45 minutes.

To Sun

Earth, one day-later

### Earth-

Figure 1.13. A solar day is longer than a sidereal day because during the time Earth rotates it also moves along its orbit around the Sun. In the interval from one noon to the next, Earth completes slightly more than one whole turn in space.

Earth, one day-later

To Sun

### Earth-

Figure 1.13. A solar day is longer than a sidereal day because during the time Earth rotates it also moves along its orbit around the Sun. In the interval from one noon to the next, Earth completes slightly more than one whole turn in space.

What motion of Earth causes the 4-minute difference between a sidereal and a solar day?_

Answer: Earth's revolution around the Sun.

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