Starship Asterisk*

Lunar Occultation of Venus

Explanation: On May 27 Venus rose as the morning star, near the waning crescent Moon in a predawn sky already full of planets. It was close on the sky to the Moon's crescent and a conjunction of the second an third brightest celestial beacons were enjoyed by skygazers around the world. But seen from locations along a track through southeast Asia and the Indian Ocean the Moon actually passed in front of Venus in a lunar occultation. In this animated gif the 75 percent illuminated disk of Venus approaches and just begins to disappear behind the sunlit southwestern lunar limb. The telescopic frames used to construct it were captured from Reunion Island in the Indian Ocean around 4:50am local time, with the Moon and Venus very close to the eastern horizon. At the time Venus was over 180 million kilometers from Reunion Island, compared to a lunar distance of a mere 400 thousand kilometers or so. About 50 minutes later Venus emerged from behind the Moon.

<< Previous APOD

This Day in APOD

Next APOD >>

It's fun to see the apparent size difference between Venus and the Moon. A math whiz could easily use the apparent size of the Moon versus the apparent size of Venus, to figure out how much farther away Venus is than the Moon, if both are the same size.

But of course Venus and the Moon are not the same size. The size difference between Venus and the Moon is similar to the size difference between the Earth and the Moon, because Venus is just a little bit smaller than the Earth:


It is also interesting to compare the brightness of Venus versus the brightness of the Moon. But I don't think the image does a very good job of showing us their true brightness difference (or albedo, which is a measure of how much of the light that hits them that is reflected by them). Because in reality, Venus is a much, much more reflective body than the Moon, but the true albedo difference is not obvious from the APOD.

Universe Today wrote:

Albedo is a measurement of the reflectivity of an object. A theoretically perfect reflecting object would have an albedo of 1, and reflect 100% of the electromagnetic radiation that falls upon it. While an object that was perfectly black and doesn’t reflect any light would have an albedo of 0. In real life, objects in the Solar System have albedo values between 0 and 1. And in the case of Venus, the albedo is 0.75.

Just for comparison, the bond albedo of the Moon is only 0.12. That’s actually pretty dark. The brightest albedo in the Solar System is Saturn’s moon Enceladus, with an albedo of 0.99. It reflects almost all of the light that falls onto it.

So Venus is perhaps this color: ███, whereas the Moon is perhaps this color: ███

I'm just guessing. Venus is probably not that bright, and the Moon may not be quite so dark. But you get my point.

Ann

a quiz on the geometry optics:
how come that the disk of Venus is 25% dark while the disk of Moon is almost 100% dark?

It may help to contemplate the configuration if I try to redo the morning skyshine that Quentin Gineys must have deleted:

Ann wrote: ↑Thu Jun 02, 2022 5:00 am interesting to compare the brightness of Venus versus the brightness of the Moon

my guess is that Moon's surface is catching a flare here because Sun's rays are grazing.
Even a black object can reflect 100% of light if the object is smooth and the angle of the incident ray is about 90°

Ann writes:

But I don't think the image does a very good job of showing us their true brightness difference (or albedo, which is a measure of how much of the light that hits them that is reflected by them). Because in reality, Venus is a much, much more reflective body than the Moon, but the true albedo difference is not obvious from the APOD.

Yes, I guess the brightness of Moon has been increased to show more detail. The Bond albedo (not bond albedo) quoted by Universe Today seems excessive. According to Wikipedia the value is 0.81±0.04.

I noticed an interesting fact in the description. That Venus was approximately 180 million kilometres away. We usually think that Venus is the closest planet to Earth, although according to planetary orbits sometimes Mars gets that distinction or sometimes even the planet Mercury. However even though the Sun isn't in the photograph, the Sun is only some 150 million kilometres away. (Maybe 152 million kilometres away since Aphelion is about a month away.) This means that the Sun is closer to us than Venus is in that photo. Now if the sun were in that photo, the Moon is just big enough to eclipse the Sun so we can imagine how much the Sun is larger than Venus!

Strange ring on Venus?
Venus seems to have a large ring whose center is towards Earth. My first thought was that it is an artifact of image processing, but then it should be affected by the dark part of Venus, and be half-moon like. But now it is undisturbed by Venus' terminator.

Ah! What a sight! If you click on picture, you can see the Occultation
start!

Ann wrote: ↑Thu Jun 02, 2022 5:00 am It is also interesting to compare the brightness of Venus versus the brightness of the Moon. But I don't think the image does a very good job of showing us their true brightness difference (or albedo, which is a measure of how much of the light that hits them that is reflected by them). Because in reality, Venus is a much, much more reflective body than the Moon, but the true albedo difference is not obvious from the APOD.

Universe Today wrote:

Albedo is a measurement of the reflectivity of an object. A theoretically perfect reflecting object would have an albedo of 1, and reflect 100% of the electromagnetic radiation that falls upon it. While an object that was perfectly black and doesn’t reflect any light would have an albedo of 0. In real life, objects in the Solar System have albedo values between 0 and 1. And in the case of Venus, the albedo is 0.75.

Just for comparison, the bond albedo of the Moon is only 0.12. That’s actually pretty dark. The brightest albedo in the Solar System is Saturn’s moon Enceladus, with an albedo of 0.99. It reflects almost all of the light that falls onto it.

So Venus is perhaps this color: ███, whereas the Moon is perhaps this color: ███

I'm just guessing. Venus is probably not that bright, and the Moon may not be quite so dark. But you get my point.

I'm not so sure about that. The perceived brightness difference between the two is about a factor of two (because of the logarithmic response of the eye). Which doesn't seem far off the mark with the transfer function used for this image sequence.

Chris Peterson wrote: ↑Thu Jun 02, 2022 1:41 pm

Ann wrote: ↑Thu Jun 02, 2022 5:00 am It is also interesting to compare the brightness of Venus versus the brightness of the Moon. But I don't think the image does a very good job of showing us their true brightness difference (or albedo, which is a measure of how much of the light that hits them that is reflected by them). Because in reality, Venus is a much, much more reflective body than the Moon, but the true albedo difference is not obvious from the APOD.

Universe Today wrote:

Albedo is a measurement of the reflectivity of an object. A theoretically perfect reflecting object would have an albedo of 1, and reflect 100% of the electromagnetic radiation that falls upon it. While an object that was perfectly black and doesn’t reflect any light would have an albedo of 0. In real life, objects in the Solar System have albedo values between 0 and 1. And in the case of Venus, the albedo is 0.75.

Just for comparison, the bond albedo of the Moon is only 0.12. That’s actually pretty dark. The brightest albedo in the Solar System is Saturn’s moon Enceladus, with an albedo of 0.99. It reflects almost all of the light that falls onto it.

So Venus is perhaps this color: ███, whereas the Moon is perhaps this color: ███

I'm just guessing. Venus is probably not that bright, and the Moon may not be quite so dark. But you get my point.

I'm not so sure about that. The perceived brightness difference between the two is about a factor of two (because of the logarithmic response of the eye). Which doesn't seem far off the mark with the transfer function used for this image sequence.

I don't understand what that means, Chris, so obviously I can't argue with you. But I found another eclipse picture where the brightness difference between the Moon and Venus seems to be greater.


Ann

Ann wrote: ↑Thu Jun 02, 2022 2:45 pm

Chris Peterson wrote: ↑Thu Jun 02, 2022 1:41 pm

Ann wrote: ↑Thu Jun 02, 2022 5:00 am It is also interesting to compare the brightness of Venus versus the brightness of the Moon. But I don't think the image does a very good job of showing us their true brightness difference (or albedo, which is a measure of how much of the light that hits them that is reflected by them). Because in reality, Venus is a much, much more reflective body than the Moon, but the true albedo difference is not obvious from the APOD.




So Venus is perhaps this color: ███, whereas the Moon is perhaps this color: ███

I'm just guessing. Venus is probably not that bright, and the Moon may not be quite so dark. But you get my point.

I'm not so sure about that. The perceived brightness difference between the two is about a factor of two (because of the logarithmic response of the eye). Which doesn't seem far off the mark with the transfer function used for this image sequence.

I don't understand what that means, Chris, so obviously I can't argue with you. But I found another eclipse picture where the brightness difference between the Moon and Venus seems to be greater.

Lunar occultation of Venus on June 19, 2020. Photo: Kelly Beatty.

---

Ann

But the question is which one most accurately reflects the actual visual appearance. An actual 6-fold difference in brightness does not appear to our eyes to be six times different.

I may be upside-down and backwards, but isn’t that the NORTHwestern part of the Moon’s limb?

Holger Nielsen wrote: ↑Thu Jun 02, 2022 6:44 am Ann writes:

But I don't think the image does a very good job of showing us their true brightness difference (or albedo, which is a measure of how much of the light that hits them that is reflected by them). Because in reality, Venus is a much, much more reflective body than the Moon, but the true albedo difference is not obvious from the APOD.

Yes, I guess the brightness of Moon has been increased to show more detail. The Bond albedo (not bond albedo) quoted by Universe Today seems excessive. According to Wikipedia the value is 0.81±0.04.

Not sure what you're saying. The 0.84 figure you're quoting is for Enceladus! Wikipedia has Venus at 0.76, which agrees with the Universe Today citation, and Wikipedia has the moon at 0.136.

gwrede wrote: ↑Thu Jun 02, 2022 11:41 am Strange ring on Venus?
Venus seems to have a large ring whose center is towards Earth. My first thought was that it is an artifact of image processing, but then it should be affected by the dark part of Venus, and be half-moon like. But now it is undisturbed by Venus' terminator.

Yes, I see a ring too. Not sure what that is.

Ann wrote: ↑Thu Jun 02, 2022 2:45 pm I found another eclipse picture where the brightness difference between the Moon and Venus seems to be greater.

Lunar occultation of Venus on June 19, 2020. Photo: Kelly Beatty.

---

now I think that the 4-exposures pic on which the posted gif was based had Moon's exposure be 4 times greater than Venus's one.
Just because the fitting of 4 frames was such as to fix Moon and move Venus

Update. No, this is not so. When I make Moon 4 times darker, the result is not realistic at all:

GeorgeNL wrote: ↑Thu Jun 02, 2022 4:30 pm I may be upside-down and backwards, but isn’t that the NORTHwestern part of the Moon’s limb?

Yes. By modern convention, Venus is disappearing behind the northwestern limb. At about 69°N, 90°W (close to the crater Pythagoras).

Chris Peterson wrote: ↑Thu Jun 02, 2022 10:14 pm

GeorgeNL wrote: ↑Thu Jun 02, 2022 4:30 pm I may be upside-down and backwards, but isn’t that the NORTHwestern part of the Moon’s limb?

Yes. By modern convention, Venus is disappearing behind the northwestern limb. At about 69°N, 90°W (close to the crater Pythagoras).

The marvelous web page https://the-moon.us/wiki/IAU_directions describes many factors that can lead to confusion in specifying lunar locations and directions, and briefly mentions some others.

I wonder if the N<—>S error in APOD’s Venusian occultation caption might have resulted from not correctly taking Réunion‘s southern hemisphere location into account. (Fellow northerners might enjoy thinking about what’s odd about the photo at https://apod.nasa.gov/apod/ap151217.html .)