Sidereal Day (sidereal day) - Unit Information & Conversion
What is a Sidereal Day?
Definition
A sidereal day is the time it takes for the Earth to complete one full rotation on its axis relative to the distant, fixed stars. It is approximately 23 hours, 56 minutes, and 4.091 seconds long. This is slightly shorter than the 24-hour solar day, which is based on the Earth's rotation relative to the Sun.
History
The concept of the sidereal day originates from ancient astronomy. Early astronomers observed that the stars appeared to return to the same position in the sky slightly earlier each day when measured against the Sun's cycle. This discrepancy led to the understanding that the Earth's rotation period relative to the background stars (sidereal) was different from its rotation period relative to the Sun (solar). Accurate measurement of the sidereal day became crucial for precise astronomical observations and timekeeping, allowing astronomers to predict the positions of celestial objects.
Common Uses
The sidereal day and sidereal time are primarily used in:
- Astronomy: To point telescopes accurately at celestial objects. Astronomical observatories use sidereal clocks to know when specific stars or galaxies will be overhead or in a particular position for observation.
- Astrophysics: For calculations involving celestial mechanics and the timing of astronomical events.
- Satellite Tracking: Used in calculations for positioning and tracking artificial satellites in Earth orbit.
- Geodesy: For precise measurements related to the Earth's rotation and orientation in space.
It is generally not used for civil timekeeping, which relies on the solar day.
Unit FAQs
How long is a sidereal day in standard time?
A mean sidereal day is approximately 23 hours, 56 minutes, and 4.091 seconds (or about 86,164.091 seconds). This is roughly 3 minutes and 56 seconds shorter than a standard 24-hour solar day.
Why is a sidereal day shorter than a solar day?
A sidereal day measures Earth's rotation relative to distant stars. A solar day measures rotation relative to the Sun. As the Earth rotates, it also orbits the Sun. After one full rotation relative to the stars (one sidereal day), the Earth has moved slightly along its orbit. It needs to rotate a little bit further (about 1 degree, taking nearly 4 minutes) to bring the Sun back to the same apparent position in the sky (completing a solar day).
Is sidereal time the same everywhere on Earth?
No, just like solar time, local sidereal time depends on your longitude. At any given moment, different locations on Earth will have different local sidereal times, corresponding to which part of the celestial sphere is currently crossing their local meridian. However, the duration of a sidereal day is the same everywhere.
Sidereal Day Conversion Formulas
To Second:
To Minute:
To Hour:
To Day:
To Week:
To Month:
To Year:
To Millisecond:
To Microsecond:
To Nanosecond:
To Decade:
To Century:
To Millennium:
To Fortnight:
To Planck Time:
To Shake:
To Sidereal Year:
Convert Sidereal Day
Need to convert Sidereal Day to other time units? Use our conversion tool.