Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: annie123 on 15/05/2015 22:31:43

Title: what is sidereal time?
Post by: annie123 on 15/05/2015 22:31:43

I was talking to my brother in England on Skype and he asked me if I knew what sidereal time was- he'd looked it up somewhere and didn't understand the explanation. I recommended this site so he may post but i doubt it  so I'm asking of anyone can explain it in 25 words or less in a way that he could understand.I tried but he wants it from an authority I think which I am not.Who takes any notice of a sister?

Title: Re: what is sidereal time?
Post by: alancalverd on 15/05/2015 22:50:31

Siderial time is time based on the rotation of the earth with respect to the distant stars. As the earth rotates, the sun appears in the same position every 24 hours (by definition of 24 hours!) but because the earth is in orbit around the sun, the distant stars appear about 4 minutes earlier each day, so astronomers use a siderial clock to keep their telescopes pointing at any particular part of the sky.

It's disturbing to go into an old observatory and hear two clocks ticking at very slightly different rates, one driving the telescopes and the other stamping the "legal" time of day on your observations. 

Title: Re: what is sidereal time?
Post by: David Cooper on 16/05/2015 19:28:33

This is one of those cases where education misleads people, leading them to think that the Earth rotates once every 24 hours, but it's moving round the sun and it has actually completed each rotation in only 23 hours 56 minutes before having to go on round for another four minutes for the same point on its surface to point back at the sun. (Four minutes is 1/360 of a day, so it builds up to a complete rotation over a year.)

If you look in a star atlas you may find the sky divided up into zones based on 24 hours, and these represent siderial time - this grid is fixed in place against the star positions and doesn't move over time. The 0h line passes through the Square of Pegasus. The 12h line passes through the pan part of the plough, if you can bear that mixed description.

Title: Re: what is sidereal time?
Post by: Pecos_Bill on 19/05/2015 01:05:46

It is all a mathematical abstraction anyway.
The heavens that we see are all views of something that happened long ago.

I heard Dr. Helen Czersky(spelling?) talking on BBC inside science about this. Did you know the image that you see in a mirror isn't "now" but from microseconds ago?

What then is "now" in either sidereal or Pacific Daylight Savings time?

Title: Re: what is sidereal time?
Post by: PmbPhy on 19/05/2015 05:56:00

Quote from: Pecos_Bill

It is all a mathematical abstraction anyway.

There's no truth in that. To be clear on this please state precisely what you mean by the phrase "mathematical abstraction" anyway. You're not even using the term in a meaningful way.

Quote from: Pecos_Bill

The heavens that we see are all views of something that happened long ago.

So what? It's hardly relevant to the subject at hand. One isn't even interested in the star being observed. All that's useful about the star is the path that the light rays take as they pass through the solar system. It gives a reference to parallel lines which are useful when determining when a full revolution on a planet has taken place.

Quote from: Pecos_Bill

I heard Dr. Helen Czersky(spelling?) talking on BBC inside science about this. Did you know the image that you see in a mirror isn't "now" but from microseconds ago?

Of course. Everyone who knows anything about light and/or the physics of optics knows this trivial fact.

Quote from: Pecos_Bill

What then is "now" in either sidereal or Pacific Daylight Savings time?

You don't know what the term "now" means? Or you don't know how daylight savings time works?

Title: Re: what is sidereal time?
Post by: evan_au on 19/05/2015 12:00:21

Quote from: Pecos_Bill

What then is "now" in either sidereal or Pacific Daylight Savings time?

This website will calculate the local sidereal time, if you enter in your location (city, longitude or current local time):
http://www.jgiesen.de/astro/astroJS/siderealClock/

In conversion from (say) Pacific to Central time, you just add or subtract a fixed number of hours.
However, the conversion from Pacific to Sidereal time changes every day (by 4 minutes).

Title: Re: what is sidereal time?
Post by: David Cooper on 19/05/2015 17:10:28

Quote from: Pecos_Bill on 19/05/2015 01:05:46

It is all a mathematical abstraction anyway.
The heavens that we see are all views of something that happened long ago.

Most of the stars we can see are close enough that they'd look the same and be in practially the same locations even if it took zero time for their light to reach us.

Quote

I heard Dr. Helen Czersky(spelling?) talking on BBC inside science about this. Did you know the image that you see in a mirror isn't "now" but from microseconds ago?

You've just undermined your previous point, because everything you see is a view of things that happened long ago.