Reverse sun motion

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John M. Haskew
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Reverse sun motion

Post by John M. Haskew » Thu Dec 03, 2015 9:20 pm

If I had Stellarium I could answer my own questions:

If I leave Tampa at sunrise and arrive in Los Angeles six hours later, will the sun appear lower in the sky or the same in L.A.?

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Re: Reverse sun motion

Post by neufer » Thu Dec 03, 2015 9:34 pm

John M. Haskew wrote:
If I leave Tampa at sunrise and arrive in Los Angeles six hours later,
will the sun appear lower in the sky or the same in L.A.?
Los Angeles time is three hours earlier than Tampa time.

In six hours, however, Los Angeles time will be three hours later.

Ergo, the sun will appear higher in the sky on arrival in L.A..
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Re: Reverse sun motion

Post by John M. Haskew » Thu Dec 03, 2015 9:48 pm

Misunderstanding: I was not clear enough:

Upon arrival in L.A., will the sun appear lower in the sky or the same as the sun appears in Tampa at the same moment? (Need Stellarium to verify.)

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Re: Reverse sun motion

Post by geckzilla » Thu Dec 03, 2015 9:55 pm

This might help you out. It's a simple web-based planetarium that your computer should be able to handle even without a video card.
http://neave.com/planetarium/
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Re: Reverse sun motion

Post by Nitpicker » Thu Dec 03, 2015 10:13 pm

When you arrive in LA, the local time will be about 9am. At that same time, the local time in Tampa will be about noon. So, the Sun will be lower over LA, than over Tampa, at that time.

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Re: Reverse sun motion

Post by BMAONE23 » Thu Dec 03, 2015 10:19 pm

The other aspect is, LAX is at 33d 56'N and Tampa is at 27d 58'N
almost exactly 6d further south and would also cause the sun to appear a little further south in the sky at LAX

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Re: Reverse sun motion

Post by John M. Haskew » Thu Dec 03, 2015 10:21 pm

Yes, thanks, Geckzilla. I had to install Adobe Flash and that planetarium seemed to work.

Now assuming I used the planetarium correctly, the sun appears lower in the sky in L.A. than it appears in Tampa at the same moment. Therefore during travel to L.A. from Tampa, the sun will appear to descend in the eastern sky. Therefore the speed of travel must be faster than the speed of rotation of the earth, correct?

(P.S. As I was writing this, I see some other posts came up.)

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Re: Reverse sun motion

Post by BMAONE23 » Thu Dec 03, 2015 10:23 pm

It takes about 4 hours and 45 minutes to fly from Tampa to LAX so If you left Tampa at 9am local (6am LAX) when you landed at LAX it would be 10:45 local so the Sun would be 1h 45m further along it's path through the daytime sky

When you land at LAX Local time will be 1hr 45min later than when you left

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Re: Reverse sun motion

Post by BMAONE23 » Thu Dec 03, 2015 10:26 pm

If you are traveling on the Concord SST traveling at 1000 mph the sun would appear stationary but if you move greater than 1000 mph the sun would appear to travel backward in the sky

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Re: Reverse sun motion

Post by geckzilla » Thu Dec 03, 2015 10:27 pm

John M. Haskew wrote:Yes, thanks, Geckzilla. I had to install Adobe Flash and that planetarium seemed to work.

Now assuming I used the planetarium correctly, the sun appears lower in the sky in L.A. than it appears in Tampa at the same moment. Therefore during travel to L.A. from Tampa, the sun will appear to descend in the eastern sky. Therefore the speed of travel must be faster than the speed of rotation of the earth, correct?

(P.S. As I was writing this, I see some other posts came up.)
Your assumption is wrong. The Sun will appear to rise, albeit more slowly than it would if one were to remain stationary, but the plane does not travel faster than Earth's rotation if it takes six hours. If the distance between Tampa and LA is about 2100 miles, one's travel time would have to be less than 2.1 hours for the Sun to appear to move backwards across the sky.
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Re: Reverse sun motion

Post by John M. Haskew » Thu Dec 03, 2015 10:41 pm

Yes, to all posters: Your comments are correct. But there is a subtle problem that is being missed to which I seek explanation:

The sun is lower in the sky in L.A. than Tampa upon trip completion. Therefore the sun must descend during the trip. However, yes, the aircraft's speed is less than the earth's rotation speed.

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Re: Reverse sun motion

Post by geckzilla » Thu Dec 03, 2015 10:50 pm

But the Sun is still higher than it was when you left; ergo, it never descended. You are confused and will figure it out.
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Re: Reverse sun motion

Post by neufer » Thu Dec 03, 2015 10:59 pm

John M. Haskew wrote:
But there is a subtle problem that is being missed to which I seek explanation:

The sun is lower in the sky in L.A. than Tampa upon trip completion. Therefore the sun must descend during the trip. However, yes, the aircraft's speed is less than the earth's rotation speed.
Therefore the sun must ascend slower during the trip than it does back in Tampa.

This is certainly true for the first half of the trip.
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Re: Reverse sun motion

Post by Nitpicker » Thu Dec 03, 2015 11:55 pm

When you fly direct from Sydney to LA on a typical commercial flight, it can take as long as 14 hours, but you arrive several hours before you leave, because you cross the dateline.

In this direction of travel, the Sun, Moon and stars appear to move faster than they would to a person at a fixed location on Earth. And, if the timing is right when you cross the equator, you can observe the Sun and the Moon switch over from being in the northward part of the sky, to the southward. (And then you get to have breakfast again. A lot of people complain about airline food, but I think it is pretty amazing to be eating a hot breakfast at 30,000 feet, whilst zooming around the planet. And since it is always cocktail hour somewhere on the planet, I like to enjoy a glass of champagne with my breakfast, too.)

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Re: Reverse sun motion

Post by John M. Haskew » Fri Dec 04, 2015 12:34 am

The vehicle for the trip does not matter. It may be a plane, a train, a car, or walk. (Suppose one can walk to L.A. in six hours.)

The observation method is simple: Measure the altitude of the sun in Tampa. Measure in New Orleans. Measure in Dallas. Measure in Phoenix. Measure in L.A.

The altitude of the sun will decrease at each location. Therefore the sun does, in fact, appear to descend during the trip.

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Re: Reverse sun motion

Post by Nitpicker » Fri Dec 04, 2015 12:40 am

John, when you left Tampa, it was sunrise, so how could the Sun descend during the trip and still be observable?

The vehicle is entirely relevant. It is only if it travels westward faster than the Earth rotates eastward, that the Sun would appear (or disappear) to move in reverse.

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Re: Reverse sun motion

Post by John M. Haskew » Fri Dec 04, 2015 12:48 am

Granted: Leave Tampa at noon and travel to L.A. in six hours. Measure at each location. The sun will appear to descend.

Absolutely correct: If (and only if) the vehicle travels faster than the earth's rotation will the sun appear to descend.

This is the problem.

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Re: Reverse sun motion

Post by Nitpicker » Fri Dec 04, 2015 12:55 am

Forget about ascending/descending, think instead of forward/reverse motion. Once you get past solar noon, the forward motion of the Sun is descending. Before noon, the forward motion is ascending. There is no problem. The Sun will still rise tomorrow.

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Re: Reverse sun motion

Post by John M. Haskew » Fri Dec 04, 2015 12:57 am

Very well: Leave Tampa at noon and travel to L.A. in six hours. Measure at each location. The sun will appear to move in reverse.

(P.S. Notice carefully the title of this thread.)

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Re: Reverse sun motion

Post by Nitpicker » Fri Dec 04, 2015 1:01 am

John M. Haskew wrote:Very well: Leave Tampa at noon and travel to L.A. in six hours. Measure at each location. The sun will appear to move in reverse.
No it won't. It will appear to move more slowly forwards than if you were not travelling. You would need to travel to LA in less than 3 hours, for the Sun to begin to appear to move in reverse.

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Re: Reverse sun motion

Post by John M. Haskew » Fri Dec 04, 2015 1:07 am

Yes, it will move in reverse.

Agreed: You would need to travel to L.A. in less than three hours for the sun to appear to move in reverse.

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Re: Reverse sun motion

Post by Nitpicker » Fri Dec 04, 2015 1:10 am

John M. Haskew wrote:(P.S. Notice carefully the title of this thread.)
Oh I did my friend. I also noticed that in your first and second post, you only mentioned the elevation of the Sun, rather than the forward/reverse motion of the Sun. That is perhaps the problem.

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Re: Reverse sun motion

Post by John M. Haskew » Fri Dec 04, 2015 1:13 am

Very well, let's start over:

Leave Tampa at noon and travel to L.A. in six hours. Measure at each location. The sun will appear to move in reverse.

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Re: Reverse sun motion

Post by Nitpicker » Fri Dec 04, 2015 1:15 am

John M. Haskew wrote:Yes, it will move in reverse.

Agreed: You would need to travel to L.A. in less than three hours for the sun to appear to move in reverse.
Um, I said it won't move in reverse, if the travel time is 6 hours. I'm done with you, Sir. Good day.

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Re: Reverse sun motion

Post by John M. Haskew » Fri Dec 04, 2015 1:17 am

True: You would need to travel to L.A. in less than three hours for the sun to appear to move in reverse.

True: Travel to L.A. in six hours. The sun will appear to move in reverse.