<p>I would like to calculate how the RA/Dec changes when an object has an apparent linear motionin the equatorial sphere. For example, more or less what an artificial satellite would do in a circular orbit.</p>

<p>In particular, what I'm looking for is as follows:</p>

<p>I have the RA/dec of the object at two times, <em>t1</em>, and <em>t2</em>. Assuming it has a linear motion at constant speed, what RA/dec will it have a <em>t3</em>? So it will have an angular separation proportional to the one it had between <em>t1</em> and <em>t2</em>, and the three points will form a straight line.</p>

CC BY-SA 3.0
2015-07-30T05:52:01.057Z
@barrycarter no, you are not missing anything. I did not know how to search for the formula in the internet even assuming that it looked like a very simple problem. I later found the formula (for Earth surface motion using lat/long but it is the same problem). Thank you,
siritinga
CC BY-SA 4.0
2020-05-13T13:36:18.043Z
@siritinga If you solved your problem, you can post an answer to your own question, in case someone else with the same problem finds this post in the future.
usernumber
<orbit><observational-astronomy>
Linear motion in the equatorial frame
siritinga
# Linear motion in the equatorial frame

(Asked by: username_0 on 2015-07-23)

I would like to calculate how the RA/Dec changes when an object has an apparent linear motionin the equatorial sphere. For example, more or less what an artificial satellite would do in a circular orbit.

In particular, what I'm looking for is as follows:

I have the RA/dec of the object at two times, *t1*, and *t2*. Assuming it has a linear motion at constant speed, what RA/dec will it have a *t3*? So it will have an angular separation proportional to the one it had between *t1* and *t2*, and the three points will form a straight line.

**Tags:** orbit, observational-astronomy

> **Comments:**
> **username_0**: @barrycarter no, you are not missing anything. I did not know how to search for the formula in the internet even assuming that it looked like a very simple problem. I later found the formula (for Earth surface motion using lat/long but it is the same problem). Thank you,  
> **username_1**: @username_0 If you solved your problem, you can post an answer to your own question, in case someone else with the same problem finds this post in the future.  

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## 0 answers

