APOD: Venus, Mercury, and Moon (2010 Apr 22)

[APOD Robot]

Venus, Mercury, and Moon

Explanation: Earlier this month, Venus and Mercury climbed into the western twilight, entertaining skygazers around planet Earth in a lovely conjunction of evening stars. Combining 8 images spanning April 4 through April 15, this composite tracks their progress through skies above Portsmouth, UK. Each individual image was captured at 19:50 UT. The sequential path for both bright planets begins low and to the left. But while Venus continues to swing away from the setting Sun, moving higher above the western horizon, Mercury first rises then falls. Its highest point is from the image taken on April 11. Of course on April 15, Venus and Mercury were joined by a young crescent Moon.

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[biddie67]

I think that somehow I arrived here before the doors officially opened ....

But what an interesting photograph - besides the amazing colors, it's pictures like this that help me understand the relative movements of objects that we see in the evening skies - thanks for a fine photo!!!!

[Graham in New Zealand]

Why is there such a large distance between the lowest and second lowest (closest to the horizon) shots of both Venus and Mercury? Thanks for the beautiful photographic work.

[zbvhs]

This is what Ptolemy and his predecessors saw and illustrates why it took so long to figure out that the planets orbit the Sun. They just saw lights moving against a star background. Mercury's path looks rather like an orbit because it's fast-moving relative to Earth but Venus' apparent motion is distorted by Earth's orbital motion. It's not at all obvious from observations such as these that our little part of the universe is Solar-centric.

[owlice]

Why is there such a large distance between the lowest and second lowest (closest to the horizon) shots of both Venus and Mercury? Thanks for the beautiful photographic work.

I'll take a guess and suggest there were two cloudy nights which prevented photographing the planets. Looks as though there was at least one more cloudy night later, too, or perhaps the photographer had something else to do those nights.

[bystander]

Why is there such a large distance between the lowest and second lowest (closest to the horizon) shots of both Venus and Mercury? Thanks for the beautiful photographic work.

I'll take a guess and suggest there were two cloudy nights which prevented photographing the planets. Looks as though there was at least one more cloudy night later, too, or perhaps the photographer had something else to do those nights.

I concur, but I think there were three nights in the initial gap and one later. There are only eight images for twelve nights, four are missing.

[owlice]

I concur, but I think there were three nights in the initial gap and one later. There are only eight images for twelve nights, four are missing.

Ah! Yes, I think you are right. Thanks!

[inspired newbie]

Can I buy this photo? As others have said, it shows us what the first astronomers were working with and illustrates the orbital movement of us in relationship to our neighbors. Great teaching tool...

Thanks!

[Dan Schroeder]

For a teaching tool (if you don't care about aesthetics), you can make images like this with the free Sky Motion Applet http://physics.weber.edu/schroeder/sky/ ... pplet.html. Set the longitude, latitude, and UT, turn on "trails", and click the date dial repeatedly to step forward in time.

[Axel]

Why does Mercury appear to change direction on the 11th of April in this picture when it went retrograde in longitude and right ascension, and changed direction in declination, around the 17th-18th iirc?

[Dan Schroeder]

Why does Mercury appear to change direction on the 11th of April in this picture when it went retrograde in longitude and right ascension, and changed direction in declination, around the 17th-18th iirc?

The multiple exposures in the photo were taken at intervals of one solar day, not one sidereal day. Retrograde motion is ordinarily defined with respect to the stars, not the sun.

[Axel]

Why does Mercury appear to change direction on the 11th of April in this picture when it went retrograde in longitude and right ascension, and changed direction in declination, around the 17th-18th iirc?

The multiple exposures in the photo were taken at intervals of one solar day, not one sidereal day. Retrograde motion is ordinarily defined with respect to the stars, not the sun.

Umm... That should not make any difference. The tables from which I get my information are based on Terrestrial Time (and I just rechecked on the IMCCE website).

[graham in New Zealand]

I concur, but I think there were three nights in the initial gap and one later. There are only eight images for twelve nights, four are missing.

Ah! Yes, I think you are right. Thanks!

Yes, thanks - that makes sense.

[DavidLeodis]

Why is there such a large distance between the lowest and second lowest (closest to the horizon) shots of both Venus and Mercury? Thanks for the beautiful photographic work.

Hi Graham. This may help. In the information with the image (which can be found in the Digitalsky website that is brought up through the Digital-Astronomy link) it states the 8 images of Venus and Mercury were taken on April 4, 8, 9, 11, 12, 13 ,14 and 15. That of the Moon was taken on the 15th.

[Dan Schroeder]

Why does Mercury appear to change direction on the 11th of April in this picture when it went retrograde in longitude and right ascension, and changed direction in declination, around the 17th-18th iirc?

The multiple exposures in the photo were taken at intervals of one solar day, not one sidereal day. Retrograde motion is ordinarily defined with respect to the stars, not the sun.

Umm... That should not make any difference. The tables from which I get my information are based on Terrestrial Time (and I just rechecked on the IMCCE website).

It most certainly does make a difference. Again, retrograde motion is defined with respect to the stars (equatorial coordinates). If the stars were visible in this photo, the would be about a degree lower each day. So when Mercury is stationary with respect to the stars, it's already moving downward with respect to the sun.

[Axel]

The multiple exposures in the photo were taken at intervals of one solar day, not one sidereal day. Retrograde motion is ordinarily defined with respect to the stars, not the sun.

Umm... That should not make any difference. The tables from which I get my information are based on Terrestrial Time (and I just rechecked on the IMCCE website).

It most certainly does make a difference. Again, retrograde motion is defined with respect to the stars (equatorial coordinates). If the stars were visible in this photo, the would be about a degree lower each day. So when Mercury is stationary with respect to the stars, it's already moving downward with respect to the sun.

Ah, right, now I see what you mean. I was confused by your reference to how retrograde motion is defined. In fact it is not defined with reference to the fixed stars; it is defined with reference to whatever coordinate system is in use for a particular case. So the turn-around in the picture is not technically a retrogradation, but a change in the distance between Mercury and the horizon at 19:50 UT. Thanks for pointing this out: it is very interesting. I am surprised that there is a week between the two phenomena - coordinate retrogradation and change of planet-horizon distance at sunset twilight.

FYI, from DE405 via IMCCE:
-- retrogradation in right ascension (apparent of date) sometime between 6:00 and 12:00 UT on 18 April 2010
-- change of direction in declination (apparent of date) sometime between 18:00 UT on 16 April and 0:00 UT on 17 April 2010
-- retrogradation in longitude (mean of date) sometime between 0:00 and 6:00 UT on 18 April 2010

[Axel]

I should add that I also checked Mercury's position in the astrometric J2000 and mean J2000 systems and there is no appreciable difference in retrogradation dates from those I posted above.

[William K.]

Excellent picture.
You will want to correct the error in the link to "Tomorrow's picture."