APOD: Mercury on the Horizon (2016 Jul 17)

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APOD: Mercury on the Horizon (2016 Jul 17)

Post by APOD Robot » Sun Jul 17, 2016 4:07 am

Image Mercury on the Horizon

Explanation: Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while after sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year, informed skygazers with a little determination can usually pick Mercury out from a site with an unobscured horizon. Above, a lot of determination has been combined with a little digital manipulation to show Mercury's successive positions during March of 2000. Each picture was taken from the same location in Spain when the Sun itself was 10 degrees below the horizon and superposed on the single most photogenic sunset. Currently, Mercury is rising higher above the horizon with each passing sunset, and just now is angularly very close to the brighter planet Venus.

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Re: APOD: Mercury on the Horizon (2016 Jul 17)

Post by Ann » Sun Jul 17, 2016 7:24 am

This is a very beautiful image. The sky colors are lovely, and it is a great composition. The little arc traced out by Mercury looks - sweet, for lack of a better word, as if Mercury was a sort of doll house planet.
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Mercury was of course named after the Latin name of the Greek god Hermes, messenger of the gods and the fastest of them all. The DC comics character The Flash is inspired by Hermes, or Mercury. I love how the original Flash, Jay Garrick, wore a perfect Hermes hat and had wings on his boots.

Of course today's APOD is a composite image. Not even Mercury, or the Flash, is fast enough to trace out a lovely sky arc like the one in today's APOD during a single sunset or sunrise. But Mercury the planet is pretty fast all the same - its mean orbital velocity is 47.36 kilometer per second, versus 29.78 km/s for the Earth. (Oh, the Flash could outrun that if he was really faster than light, or at least as fast as light - but sorry Flash, your speed still has to make it off the comic book pages, or the flickering of a silver screen or another kind of screen.)

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Re: APOD: Mercury on the Horizon (2016 Jul 17)

Post by Boomer12k » Sun Jul 17, 2016 4:19 pm

WOW, so cool.....

I won't be able to see Mercury anymore, the trees are too big, and the new home is in a woodsy area, with hills... but I should be able to find a high point and go with a smaller scope to see things... but that is in the future.

Sigh.... Darn Squirrels... Yeah, they're after me... it's a conspiracy.... I... I... I know too much....

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Re: APOD: Mercury on the Horizon (2016 Jul 17)

Post by Coil_Smoke » Sun Jul 17, 2016 6:54 pm

Wow, This is one great accomplishment. I vote this as one of the all time greatest images ever presented @ APOD.

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Re: APOD: Mercury on the Horizon (2016 Jul 17)

Post by MarkBour » Sun Jul 17, 2016 11:55 pm

This is pretty neat, and certainly a beautiful composition. Is there a name for this, as an astronomical diagram?

The images superposed were stated to be taken at the same point in the evening relative to the sun's position with the horizon. I wonder how the arc would change if the images had been taken at the exact same time of day. Probably not a huge difference over the course of a month, but I guess if the Sun is setting a little later each day, then this track would move higher. It would be quite nice to see a diagram of the movement of Mercury's same-time-of-day position over the course of a year or more. To be sensible, the diagram would have to be done with something like Geck's rendering software and would need to be plotted in a way that can show positions below the horizon. (I wonder out loud if she's interested in doing that? Maybe I should try to get that software.)
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Re: APOD: Mercury on the Horizon (2016 Jul 17)

Post by geckzilla » Mon Jul 18, 2016 12:34 am

MarkBour wrote:This is pretty neat, and certainly a beautiful composition. Is there a name for this, as an astronomical diagram?
An analemma? We only see the tip.
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Re: APOD: Mercury on the Horizon (2016 Jul 17)

Post by MarkBour » Mon Jul 18, 2016 11:58 pm

geckzilla wrote:
MarkBour wrote:This is pretty neat, and certainly a beautiful composition. Is there a name for this, as an astronomical diagram?
An analemma? We only see the tip.
Thank you! That word may be the one to use. The most basic meaning of analemma seems to be a tracing of the Sun's position in the sky from a given point on the Earth at a given time of day over the period of a year. (After which it "repeats", although there is probably subtle movement if you watch long enough?) Wikipedia suggests the term may be expanded; they have, in the article https://en.wikipedia.org/wiki/Analemma, "Although the term 'analemma' usually refers to the Earth's solar analemma, it can be applied to other celestial bodies as well." I cannot tell from just this short sentence in what sense they mean "other celestial bodies", though. I found online computed images of the analemma of Sol from other planets, such as one from a point on Mars that was a prior APOD: http://apod.nasa.gov/apod/ap061230.html.

I'm looking for a term for a diagram of the points in the sky (above or below the horizon) for any celestial object as seen from a given point on Earth at a given time of day over the course of time. Perhaps analemma can be used for this for any object. The APOD: http://apod.nasa.gov/apod/ap050713.html seems to be using analemma in just this sense, when it considers "an analemma of the Moon". Although apparently the image in that particular APOD was constructed somewhat differently.

So, much like the current APOD suggests, if we traced the points for an object such as Mars or Mercury from a point on the Earth such as Stonehenge, I assume we would get a far more involved shape than we do for the Sun's analemma. I haven't seen any such observationally-constructed diagram for a planet anywhere, yet, though this APOD is closely related. I think such a diagram would have been the foundation for discussion of such matters as the heliocentric model in ancient times. It would surely show those retrograde motions that were so vexing to natural philosophers.
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Re: APOD: Mercury on the Horizon (2016 Jul 17)

Post by Nitpicker » Tue Jul 19, 2016 4:05 am

Not that it quite relates to the APOD, but yes, I think the correct term is analemma, for any celestial body in general, from a constant place and time of day on another body. From Earth, any body beyond the Solar System, would simply scribe an analemma in the form of a line of constant declination.

But in ancient times, they didn't have accurate clocks to establish the constant time of day. The retrograde loops of the planets were still observed, of course, but relative to the "fixed" stars. Anytime on a clear night was suitable to observe. There are a few APODs that show these loops. That the loops caused such vexation, is another matter entirely.

This APOD is neither an analemma, nor a loop against the "fixed" stars, but it is similar to both. I don't know what it is called, but I like it.