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How does one measure the angle of Sun and Mars at different time? 
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#1
Jan114, 08:13 PM

P: 13

NASA's horizon system shows the angle (SOT) 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 SOT approaches 175/176/178 degrees, how does one from the view of Earth measure this angle? In general, if SOT is small (30 degrees, for instance), an instrument may be easily put to do this measurement, however, if SOT 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.



#2
Jan114, 08:44 PM

P: 119

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
Jan114, 10:21 PM

P: 13

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



#4
Jan214, 04:42 AM

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



#5
Jan214, 06:35 PM

P: 13

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 SOT 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
Jan314, 09:15 AM

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P: 39,344

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
Jan314, 10:16 AM

P: 381

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
Jan314, 11:34 AM

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#9
Jan314, 02:19 PM

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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 how did Kepler know that the time of Mars' orbit was 687 days 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
Jan314, 04:09 PM

P: 119

I think perhaps the original poster believes that the data from NASA's Horizons system is realtime 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
Jan514, 07:19 PM

P: 13

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 realtime observation. So, I always use the data to make comparison with my work. 


#12
Jan514, 07:44 PM

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#13
Jan514, 07:46 PM

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#14
Jan514, 07:57 PM

P: 13

Can you please show me where to find these observations? I through out websites but find nothing. 


#15
Jan514, 08:06 PM

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P: 22,239

Here's Tycho Brahe's Mars data, found via google: http://www.pafko.com/tycho/observe.html
What exactly do you need/what do you need it for? 


#16
Jan514, 08:16 PM

P: 13

If there are more observations, I can do a good comparison. 


#17
Jan514, 08:21 PM

P: 13




#18
Jan614, 05:58 AM

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