APOD: Daytime Moon Meets Morning Star (2015 Dec 10)

APOD Robot · Post by **APOD Robot** » Thu Dec 10, 2015 5:05 am

Daytime Moon Meets Morning Star

Explanation: Venus now appears as Earth's brilliant morning star, standing in a line-up of planets above the southeastern horizon before dawn. For most, the silvery celestial beacon rose predawn in a close pairing with an old crescent Moon on Monday, December 7. But also widely seen from locations in North and Central America, the lunar crescent actually occulted or passed in front of Venus during Monday's daylight hours. This time series follows the daytime approach of Moon and morning star in clear blue skies from Phoenix, Arizona. The progression of nine sharp telescopic snapshots, made between 9:30am and 9:35am local time, runs from lower left to upper right, when Venus winked out behind the bright lunar limb.

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FLPhotoCatcher · Post by **FLPhotoCatcher** » Thu Dec 10, 2015 6:08 am

How long did it take for Venus go from 100% brightness to 0% brightness as it occulted? If it were a star, I imagine it would be practically instant, but Venus does have some angular size to it.

DavidAH · Post by **DavidAH** » Thu Dec 10, 2015 1:32 pm

Venus appears closer to the moon in the first image on the bottom left than the second image. Are these images out of order?

Chris Peterson · Post by **Chris Peterson** » Thu Dec 10, 2015 1:51 pm

FLPhotoCatcher wrote:How long did it take for Venus go from 100% brightness to 0% brightness as it occulted? If it were a star, I imagine it would be practically instant, but Venus does have some angular size to it.

I was watching through binoculars when it disappeared. I'd estimate it faded over 5-10 seconds. Longer than I was expecting.

neufer · Post by **neufer** » Thu Dec 10, 2015 1:57 pm

FLPhotoCatcher wrote:
How long did it take for Venus go from 100% brightness to 0% brightness as it occulted? If it were a star, I imagine it would be practically instant, but Venus does have some angular size to it.

A Lunar month is about 708 hours so that the Moon moves (vis-a-vis the Sun/Venus)
~ 1º every 2 hours or ~ 1" every 2 seconds.

The width of Venus around elongation is about 10" ; so about 20 seconds of time.

Chris Peterson · Post by **Chris Peterson** » Thu Dec 10, 2015 3:12 pm

FLPhotoCatcher wrote:How long did it take for Venus go from 100% brightness to 0% brightness as it occulted? If it were a star, I imagine it would be practically instant, but Venus does have some angular size to it.

My earlier comment was that I subjectively observed the fade to occur over 5-10 seconds.

Modeling the occultation in a simulation shows the actual time from first to last contact as 43 seconds. I went back to the photo sequence I shot of the event, and the time between my first and last contact frames was 46 seconds- quite consistent with the simulation. The discrepancy between these long times and the shorter visual one is probably explained by the fact that I didn't notice the dimming until Venus was partly occulted already, and in the daylight sky it probably became invisible before it was completely occulted.

PeterH · Post by **PeterH** » Thu Dec 10, 2015 3:17 pm

If you miss the sequence as starting with the bottom left, you can actually perceive that Venus is in front of the moon which is impossible! Startling image.

heehaw · Post by **heehaw** » Thu Dec 10, 2015 4:25 pm

How beautiful it all is!

MarkBour · Post by **MarkBour** » Thu Dec 10, 2015 4:57 pm

Love this sequence and the way it was composed! (I agree, there seems to be an issue with the lower left image, perhaps it is out of sequence, as DavidAH conjectured.)

I can just make out a crater on the lunar crescent very near (visually) where Venus "set". If the Moon had an atmosphere, it would have been fun to see the color change. No doubt the Moon bent the light from Venus as it went by. I assume this relativistic effect was too small to measure. Hmmm, but it must exist ... puts a twist on the famous verification of Einstein's prediction during a solar eclipse ... the light would actually have been altered by the Sun and then the Moon.

Ron-Astro Pharmacist · Post by **Ron-Astro Pharmacist** » Thu Dec 10, 2015 7:50 pm

Today's APOD would have made a good jacket cover for Seveneves. Venus could nicely double for the "agent" that leaves Earth in a pickle likely proving one man's Venusian hell is another's Cytherian heaven. Luckily Venus really missed (by around 60 million miles??) and we live to see another day in paradise.

Chris Peterson · Post by **Chris Peterson** » Thu Dec 10, 2015 9:54 pm

Re my earlier comments about the time between first and last contact, here's a composite from beginning to end, with the frames separated by six seconds each (48 seconds total).

Ann · Post by **Ann** » Fri Dec 11, 2015 2:55 am

Can anyone educate me? When you check out the APOD for December 9, it says at the bottom of that page, Tomorrow's picture: series cytherean

Well, "tomorrow's picture" (that is, the APOD for December 10) turned out to be the daytime occultation of Venus by the Moon. But what does cytherean mean? I googled it and was none the wiser. I asked for pictures, and was shown pictures I didn't want to see. I hope my computer doesn't remember those pictures and think I want to look at similar pics in the future.

Ann

Chris Peterson · Post by **Chris Peterson** » Fri Dec 11, 2015 3:28 am

Ann wrote:Can anyone educate me? When you check out the APOD for December 9, it says at the bottom of that page, Tomorrow's picture: series cytherean

Well, "tomorrow's picture" (that is, the APOD for December 10) turned out to be the daytime occultation of Venus by the Moon. But what does cytherean mean? I googled it and was none the wiser. I asked for pictures, and was shown pictures I didn't want to see. I hope my computer doesn't remember those pictures and think I want to look at similar pics in the future.

The first thing I got from Google: https://en.m.wikipedia.org/wiki/Cytherean

Rusty Brown in Cda · Post by **Rusty Brown in Cda** » Fri Dec 11, 2015 11:22 am

DavidAH wrote:Venus appears closer to the moon in the first image on the bottom left than the second image. Are these images out of order?

That was my first thought as well. Or is there complex "orbital mechanics" at work here?
Anyone?

neufer · Post by **neufer** » Fri Dec 11, 2015 4:59 pm

[img3="Complex "orbital mechanics" "]http://headandneckcancerguide.org/wp-co ... orbit1.jpg[/img3]

Rusty Brown in Cda wrote:
DavidAH wrote:
Venus appears closer to the moon in the first image on the bottom left than the second image. Are these images out of order?
That was my first thought as well.
Or is there complex "orbital mechanics" at work here? Anyone?

Starship Asterisk*

APOD: Daytime Moon Meets Morning Star (2015 Dec 10)

APOD: Daytime Moon Meets Morning Star (2015 Dec 10)

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