Wednesday, June 4, 2008

Sci-Fi 101: Sidereal Time

Star Time Versus Sun Time

Here’s an exercise:
Go outside at night.
Find an object such as a light pole or post and pick a star it points to.
Note the time.
Look again the very next night at about the same time.
The star will arrive about four minutes earlier than it did the night before.

Here’s the question: If it takes the Earth 23 hours and 56 minutes to turn on its axis, why is a day 24 hours long?

Because if we followed the 23 hours, 56 minute day, “noon” would eventually be in the middle of the night, so we don’t follow sidereal or “star time”. We follow “solar time” or sun time.

Thinking, “Huh?”

Yeah, so did I. So I decided to unravel this mystery and what it means, because, you know, I might use it in a novel sometime. Here’s what I found out.

The unit of solar time (a day) is the time it takes our Sun to travel 360 degrees around the sky, due to the rotation of the Earth. A day can be broken down into smaller units, as such:

· 1/24 Day = 1 Hour
· 1/60 Hour = 1 Minute
· 1/60 Minute = 1 Second

But the Earth doesn’t actually spin 360 degrees in a Solar Day. Since it’s in orbit around the Sun, it moves about one degree along its orbit for every day (because 360 degrees/365.25 days for a full orbit is equal to approximately one extra degree per day). So, the Earth actually has to spin 361 degrees to make the Sun look like it has traveled 360 degrees around the sky.

In astronomy, what’s important is how long it takes the Earth to spin with respect to the much more distant stars, not our Sun. The Earth’s orbit needs to be removed as a complication of the timescale so the only consideration is how long it takes the Earth to spin 360 degrees with respect to the stars (not the Sun). This is called sidereal time, and the time span is a sidereal day. It’s approximately four minutes shorter than a solar day (because of the additional degree the Earth rotates in a solar day).

A sidereal day is defined as 23 hours, 56 minutes, and sidereal hours, minutes and seconds are the same fraction of a day (see above) as their solar counterparts. A solar second is equal to 1.00278 sidereal seconds. A sidereal clock has 24 hours with no am or pm, starting at 00, but the clock moves at a different rate than the solar clock.

Sidereal time determines where stars are at any given time. Sidereal time divides one full spin of the Earth into 24 sidereal hours. Without getting too far down in the weeds (because, Lord knows, I’d need a machete to hack my way out) the map of the sky is divided into 24 Hours of Right Ascension. Local Sidereal Time (LST) indicates the Right Ascension on the sky that is in the process of crossing the local meridian. If a star has a Right Ascension of 05h 32m 24s, it will be on your meridian at LST=05:32:24.

So why don’t we just use sidereal time instead of solar time for our clocks? Because, as mentioned above, if we did, eventually “noon” would be in the middle of the night or “morning” would be in the evening because it falls short by that one degree. We use solar time as our basis for clocks because this time scale makes more sense for daily life, where the sun rises in the morning and sets in the evening.

Interesting things to think about if your genre deals with stars, planets and time.

If you’d like to know more, check out this web site:


  1. Ok, this post is like, totally rad and timely for me because I'm in the middle of Jay Lake's MAINSPRING and "sidereal time" is part of the characters' vernacular.

    I thought the author had made it up for the story but obviously not (and I was obviously too lazy to look it up!).

    Now I understand the science in the book a lot better. Laurie, is there anything you don't know?

  2. Whoa, that's interesting. I'll have to take a look at MAINSPRING.

    Heather, actually this concept confused me, so I kept researching it until I could get a good understanding of how it works. I'm glad it was helpful to you. :)

  3. MAINSPRING has some really great world building. I appreciate you condensing the sidereal concept into an easy to digest post!


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