sidereal

The adjective sidereal means based on the celestial sphere, the apparently-fixed position of the stars. Over a short period of time (e.g., days), stars appear to us to be permanently fixed relative to each other, and sidereal refers to positions relative to those fixed positions, and over somewhat longer times (years, decades), that is approximately true.

A sidereal period is the orbital period of a body, counting as one orbit, one cycle such that the direction of the orbiting body from the host body is toward the same sidereal sky position. For example, we may think of the Moon as having completed an orbit when it is back over our head at a certain time of day (e.g., the midnight when the Moon is nearest the zenith). However, the Earth orbits a bit each day and that given time of day occurs with the Earth facing a different direction relative to the distant stars. So a lunar orbital period based upon "overhead to overhead" does not match the orbit's sidereal period; the time from overhead to overhead is termed the orbit's synodic period.

The rotation period can also be either sidereal or synodic, and the terms sidereal orbital period and sidereal rotation period may be used when there is a need to make the intended meaning clear.

Over longer periods (years, decades), a small amount of movement of some stars is measurable, and over the longest term, no astronomical objects would show fixed positions relative to each other, yet the concept of sidereal is useful to us to describe positions in the sky. Generally, the more distant the object, the less its movement is evident from the Earth, and for a stable, long-term description of a position in the sky, it is specified relative to distant Milky Way stars, galaxies, and quasars.

Sidereal time is time based upon the position of the stars, e.g., without respect to the position of the Sun and moon as we see it, or other adjustments. A sidereal year is the Earth's sidereal period (365.2422 days), a sidereal month is the Moon's (27.32166 days), and a sidereal day is the time it takes the Earth to turn until the same stars are overhead (roughly 23 hours 56 minutes 4 seconds), its sidereal rotation period.

The "ordinary" year (which our calendar year approximates with its occasional leap years with an extra day) is termed the tropical year, aligned with the seasons, which are aligned with the equinoxes, it being about 20 minutes shorter than a sidereal year. A third type of year, the anomalistic year is aligned with the ellipse of Earth's slightly eccentric orbit: i.e., from one perihelion (nearest point in time to the Sun) to the next: it is between four and five minutes longer than the sidereal year. For specifying lengthy intervals of time with precision, in sciences such as astronomy, the Julian year is used, being 31557600 SI seconds, i.e., the exact number in 365.25 24-hour (86400-second) days.