# How does one measure the angle of Sun and Mars at different time?

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1. Jan 1, 2014

### roufeng

NASA's horizon system shows the angle (S-O-T) of Sun and planet from the view of Earth. Could you please tell me how one measure the angle of Sun and Mars when opposition nearly takes place. In other words, if S-O-T approaches 175/176/178 degrees, how does one from the view of Earth measure this angle? In general, if S-O-T is small (30 degrees, for instance), an instrument may be easily put to do this measurement, however, if S-O-T is too large (nearly approach 180 degrees), because of the spherical structure of the Earth body, an instrument is very difficult to be put. For instance, when opposition nearly takes place, for an observer on Earth, if Sun is at noon, Mars must be approach midnight. At the moment it is unlikely to for the observer to put an instrument to make measure. However, if Sun is at morning (dawn), Mars must be approach evening (dusk). At the moment it is also very difficult for the observer to put an instrument to measure. Because of the unevenness of ground, I even guess nobody can use an instrument to measure an angle of 177/178/179 degrees for Sun and Mars.

Last edited: Jan 1, 2014
2. Jan 1, 2014

### tfr000

One compares their positions to the background stars, many of which have very precisely known positions.
"Unevenness of ground" - a plumb bob always points in the same direction - your instrument can reference from this. Not really very difficult, people have been doing this for hundreds, maybe thousands of years.

3. Jan 1, 2014

### roufeng

I see, that means the angle of Sun and planet in NASA horizon system is from deduced but not from direct observation.

4. Jan 2, 2014

### Staff: Mentor

What do you mean? We still have to observe where the Sun and the planet are in the sky. The background stars simply serve as a measurement device since we've already measured them. Kind of like calibrating a measurement tool using another tool.

5. Jan 2, 2014

### roufeng

Thanks a lot. Could you please show me anymore about how to undertake such a measurement?
I note that sextant
can take a scope of at most 144 degrees, so, if S-O-T is too much (larger than 144 degrees), we need to use background stars to make comparison with Sun and planet to obtain this angle. Is it right?

6. Jan 3, 2014

### HallsofIvy

Staff Emeritus
144 degrees is still larger than 90 degrees. If you find that the angle mars makes with the horizon is larger than 144 degrees, so you cannot read it on your sextant, turn around. The angle mars makes with the opposite horizon will be 180 minus the first angle so less than 90.

7. Jan 3, 2014

### snorkack

Angle with horizon alone does not give angle between Sun, Earth and Mars - you also need angle along horizon.

They can in principle be visible at the same time - Sun just over horizon, Mars just over opposite horizon. But there are obvious problems seeing and detecting Mars low in twilit sky.

8. Jan 3, 2014

### phyzguy

I'm still trying to understand why you insist on a direct measurement. What is wrong with measuring the position of the sun relative to the background stars, measuring the position of Mars relative to the background stars, and calculating the angle between them?

9. Jan 3, 2014

### D H

Staff Emeritus
Even worse, roufeng appears to be insisting on a direct measurement of both the Sun and Mars by a single observer, with both measurements taken at the same time.

That is not needed, roufeng. What's needed are lots of measurements of lots of solar system objects from lots of observers, spread out over time.

Based on what you have written in this thread and in the [thread=506262]how did Kepler know that the time of Mars' orbit was 687 days[/thread] thread, you give the appearance of having some agenda, an agenda that goes against the rules of this site. If that's the case, please revisit our rules. If it's not, I apologize in advance for accusing you of something you did not have in mind.

10. Jan 3, 2014

### tfr000

I think perhaps the original poster believes that the data from NASA's Horizons system is real-time measured data. None of it is. It is all predicted from JPL's models of the Solar System - the ones they use for navigating spacecraft and writing the Astronomical Almanac.

11. Jan 5, 2014

### roufeng

Dear all,
reading your reply lines makes me understand I have missed so much. Sorry, I know NASA's horizon' data not good. Because somebody once told me that horizon's data is good match with real observation. That means it is equal to real-time observation. So, I always use the data to make comparison with my work.

Last edited: Jan 5, 2014
12. Jan 5, 2014

### roufeng

Thanks a lot. So, how about the precise of the data from Horizons system? But if one wants to calculate the position of Sun and planets, if he does not have starting position data, where/which kind of data may be selected?

13. Jan 5, 2014

### phyzguy

Ummm, while tfr000 said that the NASA Horizon data is not real time observations, that doesn't mean that it is not "good". The orbits of the planets are known to incredible precision based on centuries of observations, so it is not necessary to have real time observations to know where they are.

14. Jan 5, 2014

### roufeng

That means that one still may use horizons data (as replacement of real measurement) to work. Is it right?
Can you please show me where to find these observations? I through out websites but find nothing.

15. Jan 5, 2014

### Staff: Mentor

16. Jan 5, 2014

### roufeng

I used a different method (non-elliptical model) to calculate the angle S-O-T (Sun-Earth-Mars) from 09/10/2000 to 08/10/2010, which accouts to 3650 position points and time span is 1 day, and make comparison with horizons system to find an average of the deviation is 2.98 degrees and the maximum is less than 10 degrees.
If there are more observations, I can do a good comparison.

17. Jan 5, 2014

### roufeng

I before saw this data link, but it is too short and too distant. From 1600 to 2010, more than 400 years, there must be many observations, but where are they kept now? If one looks through internet, you nearly cannot find anymore.

18. Jan 6, 2014

### phyzguy

Being 3-10 degrees off is a huge error. Tycho's measurements were accurate to about 1 arc minute (.016 degree). Modern observations are much more accurate than this. It sounds like your model is a poor fit to the actual motion of Mars.

19. Jan 6, 2014

### D H

Staff Emeritus
That is exactly what I meant when I earlier said "you give the appearance of having some agenda, an agenda that goes against the rules of this site."

roufeng, I am giving you the benefit of the doubt and assuming your "different method" is an N body gravitational simulation of the solar system. Anything other than that will get you banned as a crackpot.

If you are trying to implement an N body gravitational simulation of the solar system and are getting poor performance (e.g., 10 degrees), ask in some other thread what can be done to improve your accuracy. If you are doing anything but that, don't ask.