# Stars move as the night progresses

## Main Question or Discussion Point

You know how the stars move as the night progresses. Well is it the declination that is varying with time? I don't think that it's the right ascension because with that you measure the angle from the vernal equinox to the point of intersection of the star's meridian with the celestial equator.So it would be constant I think. Also the star's meridian is the path that the star actually follows isn't it??

I'm just trying to get my head around this because it's kind of confusing for me anyway.

I'd appreciate any feedback on whether I'm a complete prat or if I have just a little bit right. :)

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russ_watters
Mentor
Its neither! The dec and ra are a fixed coordinate system relative to the stars themselves. So while a certian star is always at the same ra, and dec, where those coordinates are in the sky changes.

tony873004
Gold Member
The star's altitude and azimuth are changing with time. The meridian is the imaginary line that runs from the north horizon, through the zenith (straight overhead) to the southern horizon. It intersects the celestial pole.

oh ok that makes more sense now.
So the coordinates are always the same, but because the Earth is rotating, the positions in the sky of those coordinates change.
So really the position of a star changes by 1 degree every 4 minutes, because the Earth rotates once in 24 hours, which is equivalent to 360 degrees.

SpaceTiger
Staff Emeritus
Gold Member
big man said:
So really the position of a star changes by 1 degree every 4 minutes, because the Earth rotates once in 24 hours, which is equivalent to 360 degrees.
Right, now do you know (or can you figure out) the main reason it isn't exactly 1 degree every 4 minutes?

Is it because the rotation of the Earth isn't exactly constant?

SpaceTiger
Staff Emeritus
Gold Member
big man said:
Is it because the rotation of the Earth isn't exactly constant?
A good guess, and that's one reason, but not the dominant one (it's a very tiny effect). Consider the entire motion of the earth. What is it doing other than rotating?

So is it precession that causes it?
Just out of interest, how much of an affect does that have on the time it takes the Earth to rotate?

SpaceTiger
Staff Emeritus
Gold Member
big man said:
So is it precession that causes it?
Actually, the cause is rooted simply in the fact that the earth goes around the sun. If the earth were rotating while fixed in space, the sun would rise and set exactly in accordance with the stars. However, the earth is also moving around the sun, so the sun is changing in celestial coordinates (right ascension and declination, fixed by the stars) slowly throughout the year. This means that the sun returns to the same position in the sky in a slightly longer time than the stars and our definition of a "day" will differ from the time it takes the stars to go around once. The former is called the solar day and is the one we know and love. The latter is called the sidereal day and is the other one amateur astronomers know and love.

haha ok I'm an ass. Thanks for explaining it though.
I kind of thought that precession was wrong when I put it down 'cause one cycle is roughly 26000 years so if it did have an affect it wouldn't be that great...but I think I will read up on time keeping now so that I at least know a bit about the different systems.

SpaceTiger
Staff Emeritus
Gold Member
big man said:
haha ok I'm an ass.
Not at all. It's not so obvious if you haven't already considered it. :tongue2:

vincentm
Gold Member
Thanks for the info ST, great to have you on this site. I learned something today :)

OK so I've been reading some more stuff on time and I came across the Equation of Time. There aren't actually thorough explanations of it really. The sites that I've seen say that the time from a sundial (is that apparent solar time?) can vary from civil time by 14 min 6 s (behind) to 16 min 33 s (ahead). I'd assume that this is the case no matter where you are in the world right? And really how often do they re-calculate the equation of time? We just talked about the eccentricity of the Earth's orbit in the above situation, but is it also the dominant reason for the variation in time given by the the Equation of Time?
I also saw the Equation of Time represented by an equation with hour angles. I mean I don't quite get that, but I suppose the above variation in time (14 min to 16 min) can be represented in an hour angle range...that range would be dependent on where you live right??

Sorry for asking so many questions, but I just can't believe how many things I've come across in just reading a small bit on time. I hope I at least some things right above

SpaceTiger
Staff Emeritus
Gold Member
big man said:
OK so I've been reading some more stuff on time and I came across the Equation of Time. There aren't actually thorough explanations of it really. The sites that I've seen say that the time from a sundial (is that apparent solar time?) can vary from civil time by 14 min 6 s (behind) to 16 min 33 s (ahead).
The best way to think about these things is to start with the simplest picture possible and then add complications one-by-one, observing the effects each one has. We started with the stationary, rotating earth and concluded that, in that scenario, the motions of the heavens would be very simple and the day easily defined. We then added the motion around the sun, considering that the sun shifted its position relative to the stars, making the "solar day" a different length from the "sidereal day".

Now let's consider adding some more complications. Remember that the earth's orbit is not perfectly circular, but is in fact more correctly described by an ellipse. This basically means that the sun's "shift" relative to the stars will vary throughout the year (depending on where we are in our orbit) and the difference between the solar and sidereal days will not be a constant. Part of the purpose of the equation of time is to correct for this effect throughout the year. The other purpose is to correct for the fact that the earth's rotation axis is tilted relative to the plane of its orbit.

I'd assume that this is the case no matter where you are in the world right?
Yup, though the sun's position on the sky at a given time of day will obviously vary with latitude.

And really how often do they re-calculate the equation of time?
I'm not sure how sophisticated they make the equation, but if they include all of the known effects (precession, nutation, etc.), then they shouldn't ever have to correct it.

I also saw the Equation of Time represented by an equation with hour angles. I mean I don't quite get that, but I suppose the above variation in time (14 min to 16 min) can be represented in an hour angle range...that range would be dependent on where you live right??
The time would, but I don't think that the range of variation would.

Sorry for asking so many questions, but I just can't believe how many things I've come across in just reading a small bit on time. I hope I at least some things right above
Ain't science great?

wow cheers for taking the time to explain that all.
It's making a lot more sense now.

haha I guess you learn something new every day ;)