position & time from a picture of the sky.

[michel123456]

If I take a picture of the clear night sky and label on it the stars & planets appearing, in conjunction with the phase of the moon, and show this labelled picture to an astronomer, can he find from this information only when and where the picture was taken?

[studiot]

In principle, yes.

 

The planets are quite variable and only give rough positions.

 

Wiki has a list of the best navigation stars to photograph.

 

http://en.wikipedia.org/wiki/List_of_selected_stars_for_navigation

As much information as possible about the time or time range of the photo would be very helpful.

Edited November 10, 2013 by studiot

[BusaDave9]

To a certain degree he can tell. He would need to see the horizon. The height of the north star will tell him how for north you are (your latitude).

He would have to know the date and time the picture was taken to determine your longitude.

 

To understand the answer to your question you only need to understand how the earth spins on it's axis. The axis of the earth's rotation goes through the north star. If you were to take a time laps photography of the night sky it would look like all the stars are rotating around the north star (see picture below). In the southern hemisphere it looks like all the stars are rotating around a curtain spot there too but there is no star at the southern axis.

 

Since the earth rotates once every day these stars would take one day to rotate around the north star. As the sun comes up in the morning it's brightness would block the view of the stars.

Also keep in mind that the earth obits around the sun. Six months from now the earth will be on the other side of the sun. At that time the sunlight will be blocking out all the stars that were visible at night 6 months ago.

Also when I travel south I can see more stars in the southern hemisphere. When I lived in Minnesota I could barely see all of Scorpius because it was on the horizon. If you are right on the equator the north star would be hard to see because it would be right on the northern horizon. The southern half of the sky would be the southern hemisphere. When you are right on the equator, and watched the sky all night, you would be able to see more stars than anywhere else in the world.

 

EDIT: Studiot and myself both said you would have to tell the astronomer the date in order for him to tell your longitude but now I take that back. You said you would mark the location of the planets. From that info an astronomer could tell the date that the picture was taken.

If you had another picture taken at twilight of dawn or dusk he would be able to tell what constellation the sun is in. In other words if you took enough pictures the astronomer would be able to tell the date, time and location the pictures were taken.

Edited November 10, 2013 by BusaDave9

[michel123456]

I am especially interested by the planets.

The stars are part of a background that rotates uniformly. But planets are seen under a specific configuration that is changing everyday and repeats every [I don't know how many] years. How many years indeed?

 

So if I see the picture of a specific configuration of planets upon the background of the stars, I should know roughly when the picture was taken, but is that sufficient to know when and where ?

Edited November 10, 2013 by michel123456

[BusaDave9]

The planets in the solar system orbit the sun in more or less a plane. This area of the sky is called the ecliptic. The sun moon and planets are always in the ecliptic. The planets closer to the sun take much less time to orbit the sun. For example Mercury orbits the sun 4 times every year. The earth once a year and Pluto takes 247 years to orbit the sun once.

Here is a pic of planets and the moon in the ecliptic

This next pic is the night sky spread out. This makes it look like the ecliptic is a curve but it just looks that way on this map because of the tilt of the earths axis. You can see in the above pic that the ecliptic tilts into the northern hemisphere and into the southern. That's why it looks like a curve when mapped out on a flat diagram.

Edited November 10, 2013 by BusaDave9

[studiot]

To a certain degree he can tell. He would need to see the horizon. The height of the north star will tell him how for north you are (your latitude).

He would have to know the date and time the picture was taken to determine your longitude.

 

Do you not think that with photogrammetric techniques the (simultaneous)equations of angles to known fixed points (stars) could be solved without a horizon, if there are sufficient observations?

Edited November 10, 2013 by studiot

[michel123456]

The planets in the solar system orbit the sun in more or less a plane. This area of the sky is called the ecliptic. The sun moon and planets are always in the ecliptic. The planets closer to the sun take much less time to orbit the sun. For example Mercury orbits the sun 4 times every year. The earth once a year and Pluto takes 247 years to orbit the sun once.

Here is a pic of planets and the moon in the ecliptic

eleptic1.jpg

Right.

Your picture is a good example.

At any other night, the planets will show in a different order, no?

So this picture corresponds to a single night over many years, I guess, in such a way that a good astronomer could say at what time (what day of what year) was that picture taken.

[Strange]

I assume you can determine location given the time, or time given the location. But not both.

[studiot]

 

I assume you can determine location given the time, or time given the location. But not both.

 

 

Why?

 

Are you thinking of 'the equation of time'?

[Strange]

 

Why?

 

Are you thinking of 'the equation of time'?

 

I'm afraid I don't know what the equation of time is (I'll look it up in a moment).

 

I was just thinking of the fact that the sky in London should look the same as the sky in New York in 5 hours time. So can you tell the difference between London now, or New York 5 hours later? Isn't this why accurate clocks were so important for navigation; in order to determine longitude?

Edited November 10, 2013 by Strange

[studiot]

 

I was just thinking of the fact that the sky in London should look the same as the sky in New York in 5 hours time. So can you tell the difference between London now, or New York 5 hours later? Isn't this why accurate clocks were so important for navigation; in order to determine longitude?

 

 

As I understand it, the longitude problem applies to moving objects such as ships. If you can stand still in one place and watch the stars revolve past then a star almanac will tell you when a certain star transits it's zenith (so long as you know the date which is why I said time range.). So if you can take a series of photos of particular stars at transit you can deduce the (celestial) time.

 

The equation of time relates the local time to universal or celestial time, and thus to longitude.

 

It is at least 35 years since I last did star shots so anyone with better knowledge is welcome to correct me.

[Enthalpy]

Time and longitude can't be resolved from an other if using only distant stars, sure. But Michel123456 suggests, if I get him properly, that other objects change more quickly, and this would indicate the date AND the position.

 

There must be no period over the whole Solar system, and certainly no short one if one includes Neptune and Uranus - but these are faint. A few object are synchronous in our Solar system, most are not, so there's no period at all - only conveniently long nearly-periods.

 

Then, Mercury and Venus are probably not on a random picture. Mars, Jupiter and Saturn would be good candidates - though the move slowly, so you need a very precise picture to get a date estimate, even if Mars is available. Since 1 nautical mile needs 1min accuracy at the Equator, a precise position would be hard.

 

The Moon is an excellent indicator. Not only does it change more rapidly, it also has many asynchronous movements making it non-periodic. Its distance changes, and so do the inclination of the orbit, the apparent inclination of the visible side... So an analyst smart enough would get the complete date just from it, and rather accurately if adding the position versus the stars.

 

If visible, geosynchronous satellites give all you need. Their positions are perfectly known and are not uniformly spaced, so they tell you where you are just if you catch the horizon as well. From the stars, you know then the date. It's just that satellite pictures prefer a camera moving with Earth while star pictures prefer to compensate this movement.

 

This one with satellites would have a manageable complexity, Moon less so, and Mars+Jupiter would demand serious detective skills.

[studiot]

 

Time and longitude can't be resolved from an other if using only distant stars,

 

Why not?

 

A given star only transits its zenith over one point at any one time.

[Strange]

 

A given star only transits its zenith over one point at any one time.

 

I don't understand that. If I see star X at its zenith "now", does that mean you can tell me both where I am and the (local) time?

 

Or is this related to the point about making observations over a period of time? I can (sort of) see how that might work. (I don't claim to understand much of this stuff.)

 

But that doesn't seem to relate to the OP, which was about a single picture (wasn't it).

Edited November 11, 2013 by Strange

[studiot]

 

I don't understand that. If I see star X at its zenith "now", does that mean you can tell me both where I am and the (local) time?

 

Or is this related to the point about making observations over a period of time? I can (sort of) see how that might work. (I don't claim to understand much of this stuff.)

 

But that doesn't seem to relate to the OP, which was about a single picture (wasn't it).

 

 

michel's original post mentioned 'a picture' and asked rather vaguely 'where and when?'

 

What did michel have in mind?

A picture would distinguish between northern and southern hemispheres and modern times and Roman times.

 

Many pictures could provide more information, but cameras are not accurate surveying instruments so ascension and declination would have to be estimated in some way from the picture(s).

 

There have been a couple of responses that suggest it is impossible to determine position and time on the planet from astro obs alone.

 

Difficult yes, impossible, no.

 

Accurate knowledge of times is very very helpful, but not essential. Years ago, surveyors used astro time obs to correct their chronometers.

[BusaDave9]

I was just thinking of the fact that the sky in London should look the same as the sky in New York in 5 hours time. So can you tell the difference between London now, or New York 5 hours later? Isn't this why accurate clocks were so important for navigation; in order to determine longitude?

yes, you are right.

 

I disagree with enthropy on this one. He seems to be saying the planets and moon would be not be moving fast enough against the star background to tell time but that's not how to tell time from the stars.

All the stars appear to rotate around the earth once each day due to the rotating Earth. As I watch the night sky and compare it to star charts I can tell what time it is knowing my location. If I know the time I can tell my location.

 

By looking at the position of the planets you can tell the date. It takes 29 years for Saturn to orbit the sun but if you also take into account the position of the other planets you can tell the year and even the time of the year. Next if you take into account the moon's position and phase you can tell pretty much what night and time the picture was taken. The stars will tell you more accurately what time it is.

 

The original post says your photo would be marked with the planets like this:

If you could even mark the distant planets such as Uranus and Neptune then an astronomer could tell you the date the picture was taken within thousands of years. He could say that picture is from June 14th 1275 BC (as if they had cameras back then)

Each planet is an accurate time keeper. The moon takes a month to orbit the sun and each planet takes a different amount of time depending on their distance from the sun. Even if you stuck with the planets that are visible with the naked eye an astronomer could tell you within hundreds of years when the picture was taken.

The height of Polaris is your latitude. While it may be hard to tell the angular height of the star from a photograph you can tell by the other stars. What star is near the horizon? Where is Cassiopeia? We know how far these stars are from Polaris so we can tell our latitude from whatever stars are below Polaris.

Edited November 11, 2013 by BusaDave9

[michel123456]

What did michel have in mind?

[deleted]

 

Never mind.

 

Edited November 11, 2013 by michel123456

[michel123456]

---------------

Well I was thinking that if one had a kind of mechanism that replicates the motion of the planets and especially their relative motion, he could use this mechanism to compare to what he observes in the sky. Making the mechanism coincide with observation would give him time because there is no 2 times with the same relative position of the planets. I was wondering if this system could also give the position.

(...) The moon takes a month to orbit the sun (...)

You surely don't mean that.

[BusaDave9]

 

You surely don't mean that.

 

I surely don't.

The moon takes a year to orbit the sun riding on the coat-tails of earth.

The moon takes a month to orbit the earth.

[Enthalpy]

[...] Making the mechanism coincide with observation would give him time because there is no 2 times with the same relative position of the planets. I was wondering if this system could also give the position.

Yes. My worry is about the accuracy of time determined that way.

 

One nautical mile corresponds to one arc minute (oops) at the Equator, or 15 seconds of time, so depending on how accurate a position is wanted, it can demand a quite accurate time - historically a challenge to navigation.

 

I strongly doubt that the relative positions of Mars and Jupiter determine the date to a few seconds, since the planets move too slowly in our observable sky. That's why I prefer our Moon, which move faster versus the stars but still gives many, unsynchronized time indications. But its fastest indication, its motion relative to the stars, is ~30 times slower than Earth rotation, so the nautical mile needs 2 arc seconds accuracy in the position of the Moon versus the stars. Not obvious.

 

My preferred one is the geosynchronous satellites, which give an excellent accuracy, provided your telescope is big enough to make a simultaneous hence brief picture of the satellites and the stars.

 

Meanwhile, I realize it's simpler. With a modest telescope that rotates with Earth, take a picture of the geosynchronous satellites and the horizon (rather, use the telescope as a sextant). This gives your position: latitude and longitude. Knowing your position, the Moon and the stars give the time.

 

What kind of "picture" do you mean? Compact digital camera, with unknown image distortion? Or a sextant, a telescope...?

[BusaDave9]

I strongly doubt that the relative positions of Mars and Jupiter determine the date to a few seconds, since the planets move too slowly in our observable sky. That's why I prefer our Moon, which move faster

No one can tell time from the planets position against the stars. You can tell the date from the planets and their position. You can tell the time from the stars. It takes only a few minutes or so for a star to set.

Re-read my post #16 on the last page. I explained this there.

 

 

 

What kind of "picture" do you mean? Compact digital camera, with unknown image distortion? Or a sextant, a telescope...?

Yes, even a compact digital camera with unknown distortion would work. You would need to see the horrizons.

The real issue is you would need a picture of polaris and the stars below it.

You would need pictures of the planets and pictures of the horizon near the ecliptic.

Or one fisheye lens photo could be enough.

Edited November 12, 2013 by BusaDave9

[michel123456]

(...)

 

What kind of "picture" do you mean? Compact digital camera, with unknown image distortion? Or a sextant, a telescope...?

O.K. let's move the goalpost a little bit.

 

It is not a picture, it is live.

You are observing the night sky for several nights from a standard position on the ground. You know the date because you have a calendar, you don't know the exact time because you don't have a clock because you are in 100 B.C. You have no telescope, all observations are with naked eyes. Also, you believe that the Earth is the center of the world.

You need to find your position because you are lost on a island or because you want to draw a map.

Is that achievable?

Hint: you have at your disposal a mechanism that replicates the apparent motion of the planets, the sun, the moon, and the phases of the moon.

[studiot]

 

O.K. let's move the goalpost a little bit.

 

 

 

Good luck!

[BusaDave9]

O.K. let's move the goalpost a little bit.

 

You need to find your position because you are lost on a island or because you want to draw a map.

Is that achievable?

 

Yes, for 600 years people have been able to navigate and find their location on earth from the stars and other heavenly bodies. Here is one link concerning celestial navigation: http://en.wikipedia.org/wiki/Celestial_navigation

You need to have a timepiece for most forms of celestial navigation. If you know the time you can find your location.

In the 1800's lunar distance navigation was invented and you no longer needed to know the time to find your location.

[Enthalpy]

No one can tell time from the planets position against the stars. You can tell the date from the planets and their position. You can tell the time from the stars. It takes only a few minutes or so for a star to set.

Stars "move" quickly with Earth's rotation, but that won't tell the time AND the position.

To get both, you need different movements that don't result from Earth's rotation. For that, I claim that planets move too slowly against the stars or against another, so the date or time obtained that way is inaccurate, and this results in a very wrong position.

Hence my preference for the Moon or for geosynchronous satellites.