degrees of movement in 1 hour

[telesto]

Hi all

If i point my scope to the east to look at a object at 40 degrees altitude on the scale of my scope how many degrees will that object move in one hour ???

regards

clive

[kniclander]

about 15

[Merlin66]

Exactly 15 degree..

[lukebl]

It depends on the declination of the object. Something on the celestial equator would move about 15 degrees in one hour (360/24), but something at the north celestial pole wouldn't move at all. Something directly east at UK latitudes at an altitude of 40 degrees would have a declination of about +25 degrees, and I guess would move about 13 degrees in an hour. To know the precise figure, you'd need to specify the declination of the object.

[Merlin66]

Sorry,

the earth's rotation causes a movement of 15 degrees per hour, no matter where you are... the distance covered by 15 degrees may appear to vary due to the closeness to the pole, but it's still 15 degrees.

[kniclander]

i would say about 15 due to the effect of the earth's movment against the background sky as well as its rotation on its own axis. no?

[lukebl]

Sorry,

the earth's rotation causes a movement of 15 degrees per hour, no matter where you are... the distance covered by 15 degrees may appear to vary due to the closeness to the pole, but it's still 15 degrees.

Perfectly true, but I assumed the question related to how far the object will have visually moved in the sky in one hour. Obviously something close to the celestial equator will move a lot further in that time than something nearer the pole.

[telesto]

Thanks for the replys folks the reason im asking is if i point my scope east set with a compass and at 40 degrees leave it an hour on track it moves 8 degrees then if i move it back to were it started set track slow it moves 7 degrees in an hour, so at these speeds it wont track an object then ??

thanks

clive

[lukebl]

'fraid I don't know anything about auto tracking dobs, but I'm sure someone will chip in.

The confusion with angles is that we have two measurements which use the same terminology. Obviously, in one hour, the object will have moved 15 degrees of its complete 360 degree circuit of the celestial pole. Or, to put it another way, one hour in Right Ascension or one 24th of its diurnal circuit. That is clearly true for every object in the sky, whether it is near the pole or near the equator.

However, the actual measurable visual distance the object has moved in terms of angular distance (For example, you might use a fist held at arms length to roughly measure a visual distance of about 10 degrees of arc) will vary, from 15 degrees for something at the celestial equator to zero degrees for something at the pole.

[Merlin66]

Whoaaa,

You're talking about Alt-Az movements not RA-Dec movements...

The Azimuth degees will vary with the declination of the object; on the equator it will still be 15 degrees per hour but the Altitude will obviously vary...( I think that's what Luke was infering)

Your starting point of 40 degrees doesn't mean much unless we know your latitiude)

[kniclander]

... but not exactly 15 degrees cos of the movement of the earth against the backgroung stars. if it was exactly 15 degrees then we'd see the same stars in the same place every night. I think...

[telesto]

Whoaaa,

You're talking about Alt-Az movements not RA-Dec movements...

The Azimuth degees will vary with the declination of the object; on the equator it will still be 15 degrees per hour but the Altitude will obviously vary...( I think that's what Luke was infering)

Your starting point of 40 degrees doesn't mean much unless we know your latitiude)

hi starting to understand now my latitude is 53 degrees

thanks everyone for your replys

clive

[Islander]

The stars shift over time because the sidereal period isn't exactly 24 hours - it's 23 hours 56 minutes 4 and a bit seconds

[Photosbykev]

and it is us doing the moving/rotating relative to the stars

[Merlin66]

OK relative to the stars (siderial) is 360 degrees in 23h 56m 4.2 s

- this works out to:

1 hour equals 15.04 degrees per hour of time

[cgarry]

... but not exactly 15 degrees cos of the movement of the earth against the backgroung stars. if it was exactly 15 degrees then we'd see the same stars in the same place every night. I think...

Correct, it is not exactly 15 degrees because the earth rotates completely in about 23 hours 56 minutes, not 24 hours.

Cheers,

Chris

[Merlin66]

No, the earth rotates at 24h per revolution....

relative to the stars, because we also rotate around the Sun, is 23h 56m 4.2s

[kniclander]

Correct, it is not exactly 15 degrees because the earth rotates completely in about 23 hours 56 minutes, not 24 hours.

Cheers,

Chris

I don't think that's right. the earth rotates completely on its own axis in 24 hours but it takes slightly less time to rotate completely against the backgrouns stars because as well as rotating on its own axis it is also moving around the sun. I think...? That's why the stars appear to rise 4 minutes earlier each evening.

[cgarry]

No, the earth rotates 360 degrees in 23h 56m 4.2s. 24h is how long it takes to face the sun again which is greater than 360 degrees because of the earth moving in its orbit.

[Islander]

Exactly. the diurnal period (rotation relative to the sun) is 24 hours, the sidereal period (rotation relative to the stars) is 23h 56m 4.1s

Yargh! edited to correct!

[kniclander]

I stand corrected

[cgarry]

This stuff can make your head hurt!

[lukebl]

Obviously when I said than an object at the celestial equator would move 15 degrees in one hour I meant nearly 15 degrees, as it is moving at the sidereal rate which, as has been pointed out, is slightly less than 24 hours.

Head hurts too...I'll get me coat.

[telesto]

This stuff can make your head hurt!

Its been making my head hurt for two weeks now

[JamesF]

I think the bottom line however is that your original question contains insufficient information to be able to answer accurately, notably how you are measuring the angles in the first place.

If you take the position of the object at the start and end of the hour as two points of a triangle, then the third point could be you, or it could be the centre of the earth (or perhaps other things too). Which is most useful depends on the circumstances.

Also, you might measure the original "height" of the object relative to your local horizon, or relative to, say, zero degrees longitude, or using "right ascension", which is measured from the point at which the sun crosses the celestial equator on 21st March (I think?). Normally RA is measured in hours, minutes and seconds though.

Even having worked that lot out, you may get different answers depending on your latitude

As you have a dob, I'd guess that you're talking about an initial position of 40 degrees above your local horizontal and that you're interested in the angle through which the tube will have to move in both alt and az to track the target for an hour?

James