What is Sidereal Time?

You’ve heard of standard time, aka solar time, but what the heck is sidereal time?  Isn’t time just time/ Yes it is but its all about how you measure it.

Simply put, it is responsible for measuring the rotation of the Earth relative to the stars instead of the sun. Astronomers use it for time keeping without referencing the Earth’s motion around the Sun. Sidereal time is also used for satellite related time calculations, where a type of universe time is needed instead of the one that is purely Sun or Earth based.

Sidereal time is also how astronomers define the rotations periods of other planets.  By using sidereal time, you can figure out how fast the planet is actually spinning regardless of its rotation around the sun.  Most of our planets have a sidereal day that is pretty close to their solar day.

But there are some interesting exceptions to that which point out why sidereal time is important in astronomy.  For the planet Mercury, its rotation takes 2/3 of the time it takes for it to orbit around the sun. So, a sidereal day for Mercury is 58 while its year is 88 days.  That means that it takes 170 earth-days to go from noon of one day to the next day. So, in fact a solar day is longer than a year on Mercury. In fact, a mercury year is about ½ of it its measured solar day.  

Venus is even more strange.  It actually rotates around the sun faster than it spins on its axis and thus is the slowest spinning planet in our solar system.  And to top it off it also spins backwards and thus the sun rises in the west and sets in the east. The sun actually returns to the highest point in the sky before a complete rotation of the planet.   If you lived on Venus, you would only enjoy two sunrises and sunsets a year.

In solar time (the time we use every day) a sidereal day is exactly 23 hours, 56 minutes and 4.0905 seconds.  In the past sidereal time was measured by observing stars with instruments such as photographic zenith tunes and Danjon astrolabes.  The movement of the stars would be timed relative to local clocks. As technology advanced technology such as radio astronomy was used as well as other methods such as the timing of pulsars.  In fact, these new technologies have led to new more precise standards for measuring the Earth’s rotation such UT1 and Earth Rotation Angle (ERA).

Determining Solar and Sidereal Time

Solar time is measured according to the movement of Earth around the sun. A mean solar day is a time between local solar noons. In a sidereal day, earth completes one rotation around its axis. It moves a short distance around 1 degree along Sun’s orbit. After one sidereal day, the earth will move slightly more to reach sun’s local noon according to local time. That is what adds 4 minutes more to the local time – it takes just a bit longer to go around the sun than it does for the earth to spin.

Also, when the solar model is measured, the only object that can be identified (based on time) is the sun; i.e. it’s directly above us at mid-day, and directly below us at mid-night. For Sidereal time it’s all about the position of the start relative to the rotation of the planet.  In other words, it’s the time it takes for the starts to be in the exact same position each day. That’s how long it takes for the earth to rotate.

In Conclusion

The rotation of the earth is definitely some independent phenomena from the rotation of the Earth around the sun.  Sidereal time allows astronomers to keep time with out also having to reference the sun. This provides another way to chart earth’s movement and align events according to these times. Without sidereal time, modern astronomy wouldn’t be what it is today.