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View Full Version : How Long is a Day on Mercury?



Fraser
2012-Dec-28, 01:50 AM
1/3 closer to the Sun than Earth is, it should be no surprise that a day on Mercury is a real scorcher, with temperatures soaring to almost 400 C. But in addition to its solar proximity it also has an extremely slow rotation: a single day on Mercury is 58.6 Earth days long…*and you thought [...]

More... (http://www.universetoday.com/99170/how-long-is-a-day-on-mercury-2/)

grapes
2012-Dec-28, 04:05 AM
Rather than "1/3 closer to the Sun than Earth", I'd've said "1/3 the distance of Earth to the Sun". The former seems to mean 2/3 the distance, to me.

ETA: I might even have said "2/5 the distance"

ASTRO BOY
2012-Dec-28, 08:22 AM
I remember a time [giving my age away here] that Mercury was once thought to be the smallest planet [at a time when Pluto was a planet] and it was gravitationally locked to the Sun with one side completely facing it and one eternally facing away....The rotational period was thought the same as the orbital period, giving what we thought were far higher and lower estimations of temperatures on both sides.
Not too far back either...just before the space age began in the mid fifties.

Frank Merton
2012-Dec-28, 09:07 AM
The rotational period was thought the same as the orbital period, giving what we thought were far higher and lower estimations of temperatures on both sides.Yea, and a couple of my high-school time books even went into detail about observations of Mercury that proved it. It seems the observers were too much influenced by the theorists who had presumed a gravitational lock after the model of the moon. There is perhaps a lesson in this.

Paul Beardsley
2012-Dec-28, 11:29 AM
Yea, and a couple of my high-school time books even went into detail about observations of Mercury that proved it. It seems the observers were too much influenced by the theorists who had presumed a gravitational lock after the model of the moon. There is perhaps a lesson in this.

Yes there is. The lesson is that it is possible to build on perfectly reasonable assumptions that happen to be wrong.

grapes
2012-Dec-28, 12:29 PM
Rather than "1/3 closer to the Sun than Earth", I'd've said "1/3 the distance of Earth to the Sun". The former seems to mean 2/3 the distance, to me.

ETA: I might even have said "2/5 the distance"
The post has been changed. Now that I look at it, even better than 2/5 would be 3/8--or better yet 5/13. :)

ETA: or, re:wiki, 12/31
http://en.wikipedia.org/wiki/Mercury_(planet)


Yea, and a couple of my high-school time books even went into detail about observations of Mercury that proved it. It seems the observers were too much influenced by the theorists who had presumed a gravitational lock after the model of the moon. There is perhaps a lesson in this.
IIRC, the explanation was that observations were consistent with tidal lock, because this particular resonance presented the same face to earth, each time, at the time. How'd that work?

Glom
2012-Dec-28, 12:39 PM
But how do you measure a day with a double sunrise?

Frank Merton
2012-Dec-28, 12:53 PM
IIRC, the explanation was that observations were consistent with tidal lock, because this particular resonance presented the same face to earth, each time, at the time. How'd that work?Ah yes, you refresh my memory. Because of the resonance, Mercury presents the same face to us whenever it is in a readily observable place in the sky, and even then earth-bound observations are extremely difficult. (Most of the time Mercury is either behind or too near the sun for us to readily observe it).

grapes
2012-Dec-28, 02:47 PM
But how do you measure a day with a double sunrise?
No double sunrise, the article is wrong, a day actually lasts 176 earth days.

The rotation time is not the same as the length of day, of course. The earth spins 366 times per revolution, but "loses" a day by going around the sun. Mercury spins three times in two years, and "loses" two days--so the length of each day is two years long.

ETA: it's possible, because of liberation, that there could be points on Mercury that experience a sort of double sunrise--the sun might come up, go back down, then right back up, but I don't know...have to do some thinking on that one. :)

Paul Beardsley
2012-Dec-28, 04:12 PM
liberation

Tee hee!

(At least you didn't put "libation"!)

Glom
2012-Dec-28, 05:11 PM
It's to do with the eccentricity of the orbit that allows Mercury to briefly be revolving faster than it rotates AIUI.

crosscountry
2012-Dec-28, 08:12 PM
No double sunrise, the article is wrong, a day actually lasts 176 earth days.

The rotation time is not the same as the length of day, of course. The earth spins 366 times per revolution, but "loses" a day by going around the sun. Mercury spins three times in two years, and "loses" two days--so the length of each day is two years long.

ETA: it's possible, because of liberation, that there could be points on Mercury that experience a sort of double sunrise--the sun might come up, go back down, then right back up, but I don't know...have to do some thinking on that one. :)


I didn't calculate it, but Starry Night did some work for me. Here's a link to a short video I made of a double sunset. You can see the Earth date and time in upper left. You can watch the sun go down, come back up a few degrees, and go down again for good.

http://www.astro.crosscountryadventures.us/Mercury_Double_Sunset/double_sunset.html

I wasn't thinking of a double sunrise, but that wouldn't be too difficult. The coordinates are ~6 South, 85E.

grapes
2012-Dec-28, 09:33 PM
Tee hee!

(At least you didn't put "libation"!)
Wow, I actually typed "libration," and that's what spell check did to me. It does have a sense of humor, I've seen.

It's to do with the eccentricity of the orbit that allows Mercury to briefly be revolving faster than it rotates AIUI.
Sorry, I misunderstood, thinking it had to do with the article calculations. Of course, you are are right.

That doesn't affect how long the "day" is though, for the same reason that an earth day doesn't vary from 24 hours, even though the time from sunrise to the next sunrise varies.