APOD: Earth and Moon (2017 Dec 04)

[APOD Robot]

Earth and Moon

Explanation: On rare occasions, the Earth and Moon are photographed together. One of most spectacular times this occurred was 25 years ago this month when the Jupiter-bound Galileo spacecraft zoomed past our home planetary system. Then, robotic Galileo watched from about 15-times the Earth-Moon separation as our only natural satellite glided past our home world. The featured video combines 52 historic color-enhanced images. Although our Moon may appear small next to the Earth, no other planet in our Solar System has a satellite so comparable in size . The Sun, far off to the right, illuminated about half of each sphere, and shows the spinning Earth's white clouds, blue oceans, and tan continents. Tonight, a nearly full Oak supermoon will be visible from all of Earth from sunset to sunrise.

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[Boomer12k]

And a LOVELY, Oak Supermoon it was to be sure... as I saw it... and took my own pictures with my new ZWO camera... but still need some testing, though the focus on this one is not too bad... about 10 frames stacked...

Happy Holiday Season...and Roll on SUPERMOON!!

:---[===] *

[heehaw]

Wow! Took me a moment (plus reading the caption) to realize that the apparent motion of the Moon is really the motion of the spacecraft!

[Lasse H]

"...no other planet in our Solar System has a satellite so comparable in size "

No proper planet in today's nomenclature, but Charon is still much bigger compared to Pluto, than Moon to Earth.

[kjhart0133]

I immediately knew something was funny about the moon's motion. heehaw's explanation makes sense, but I'm still having problems visualizing the relative motions of all three objects (earth, moon, satellite).

[Chris Peterson]

"...no other planet in our Solar System has a satellite so comparable in size "

No proper planet in today's nomenclature, but Charon is still much bigger compared to Pluto, than Moon to Earth.

Charon is a satellite by IAU declaration- the same process that classifies Pluto as a non-planet. But the barycenter of the Pluto-Charon system lies outside of Pluto, which makes it very reasonable to view the pair as binary planets, and indeed, the IAU has it on the table to review whether Charon should be reclassified as a dwarf planet like Pluto, rather than as a satellite of Pluto.

[neufer]

[b][color=#0000FF]An image of Earth taken by Galileo during the GOPEX test, showing laser pulses coming from a transmitting telescope on the night side. Galileo's imager was panned downward during the exposure to separate the pulses, thus blurring Earth's image on the right.[/color][/b]

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Galileo Optical Experiment

<<In December 1992, during Galileo's second gravity-assist planetary flyby of Earth, another groundbreaking experiment was performed. Optical communications in space was assessed by detecting light pulses from powerful lasers with Galileo's CCD. The experiment, dubbed Galileo Optical Experiment or GOPEX, used two separate sites to beam laser pulses to the spacecraft, one at Table Mountain Observatory in California and the other at the Starfire Optical Range in New Mexico. The Table Mountain site used a frequency doubled Neodymium-Yttrium-Aluminium garnet (Nd:YAG) laser operating at 532 nm with a repetition rate of ~15 to 30 Hz and a pulse power (FWHM) in the tens of megawatts range, which was coupled to a 0.6 m Cassegrain telescope for transmission to Galileo. The Starfire range site used a similar setup with a larger, 1.5 m, transmitting telescope. Long exposure (~0.1 to 0.8 s) images using Galileo's 560 nm centered green filter produced images of Earth clearly showing the laser pulses even at distances of up to 6 million km. Adverse weather conditions, restrictions placed on laser transmissions by the U.S. Space Defense Operations Center (SPADOC) and a pointing error caused by the scan platform acceleration on the spacecraft being slower than expected (which prevented laser detection on all frames with less than 400 ms exposure times) all contributed to the reduction of the number of successful detections of the laser transmission to 48 of the total 159 frames taken. Nonetheless, the experiment was considered a resounding success and the data acquired will likely be used in the future to design laser "downlinks" which will send large volumes of data very quickly from spacecraft to Earth. The scheme is already being studied (as of 2004) for a data link to a future Mars orbiting spacecraft.>>

[MarkBour]

"...no other planet in our Solar System has a satellite so comparable in size "

No proper planet in today's nomenclature, but Charon is still much bigger compared to Pluto, than Moon to Earth.

Charon is a satellite by IAU declaration- the same process that classifies Pluto as a non-planet. But the barycenter of the Pluto-Charon system lies outside of Pluto, which makes it very reasonable to view the pair as binary planets, and indeed, the IAU has it on the table to review whether Charon should be reclassified as a dwarf planet like Pluto, rather than as a satellite of Pluto.

But then that pushes the statement in the APOD caption into the realm of semantics rather than what at first sounded like an intriguing fact. (If we had 10 cases in the solar system of moons larger than ours, relative to the planet, and we kept deciding to call those instances of binaries or trinaries, etc.)

[Chris Peterson]

"...no other planet in our Solar System has a satellite so comparable in size "

No proper planet in today's nomenclature, but Charon is still much bigger compared to Pluto, than Moon to Earth.

Charon is a satellite by IAU declaration- the same process that classifies Pluto as a non-planet. But the barycenter of the Pluto-Charon system lies outside of Pluto, which makes it very reasonable to view the pair as binary planets, and indeed, the IAU has it on the table to review whether Charon should be reclassified as a dwarf planet like Pluto, rather than as a satellite of Pluto.

But then that pushes the statement in the APOD caption into the realm of semantics rather than what at first sounded like an intriguing fact. (If we had 10 cases in the solar system of moons larger than ours, relative to the planet, and we kept deciding to call those instances of binaries or trinaries, etc.)

Well, in some sense definitions always come down to semantics. But this is a distinction with a solid physical basis, and consistent with how we use the terms in a more general sense. It has nothing to do with relative sizes (except indirectly); the distinction is whether the barycenter lies outside the pair. In the case of the Earth and Moon, the barycenter is inside the Earth, so the Moon is reasonably seen as a satellite. In the case of Pluto and Charon, the barycenter lies between the two, so they are reasonably seen as binary objects. That's how we do it with asteroids, for instance.

[MarkBour]

Charon is a satellite by IAU declaration- the same process that classifies Pluto as a non-planet. But the barycenter of the Pluto-Charon system lies outside of Pluto, which makes it very reasonable to view the pair as binary planets, and indeed, the IAU has it on the table to review whether Charon should be reclassified as a dwarf planet like Pluto, rather than as a satellite of Pluto.

But then that pushes the statement in the APOD caption into the realm of semantics rather than what at first sounded like an intriguing fact. (If we had 10 cases in the solar system of moons larger than ours, relative to the planet, and we kept deciding to call those instances of binaries or trinaries, etc.)

Well, in some sense definitions always come down to semantics. But this is a distinction with a solid physical basis, and consistent with how we use the terms in a more general sense. It has nothing to do with relative sizes (except indirectly); the distinction is whether the barycenter lies outside the pair. In the case of the Earth and Moon, the barycenter is inside the Earth, so the Moon is reasonably seen as a satellite. In the case of Pluto and Charon, the barycenter lies between the two, so they are reasonably seen as binary objects. That's how we do it with asteroids, for instance.

And if our Moon were twice its present mass, it would have moved the barycenter outside of Earth and you would call it a binary.

[Chris Peterson]

But then that pushes the statement in the APOD caption into the realm of semantics rather than what at first sounded like an intriguing fact. (If we had 10 cases in the solar system of moons larger than ours, relative to the planet, and we kept deciding to call those instances of binaries or trinaries, etc.)

Well, in some sense definitions always come down to semantics. But this is a distinction with a solid physical basis, and consistent with how we use the terms in a more general sense. It has nothing to do with relative sizes (except indirectly); the distinction is whether the barycenter lies outside the pair. In the case of the Earth and Moon, the barycenter is inside the Earth, so the Moon is reasonably seen as a satellite. In the case of Pluto and Charon, the barycenter lies between the two, so they are reasonably seen as binary objects. That's how we do it with asteroids, for instance.

And if our Moon were twice its present mass, it would have moved the barycenter outside of Earth and you would call it a binary.

Yes. And that would be a reasonable distinction on purely physical grounds.

[mr.h8thsci]

My 8th science class would to know...what phase of the moon would you say is visible at about 5 seconds of the clip. We are having a tough time with the hole rotation of Earth vs. revolution of the moon. Thanks!

[Chris Peterson]

My 8th science class would to know...what phase of the moon would you say is visible at about 5 seconds of the clip. We are having a tough time with the hole rotation of Earth vs. revolution of the moon. Thanks!

The same phase that is seen during the entire clip. There's no substantive change in the lunar phase during this brief video.

[Catalina]

If it is the motion of the spacecraft moving past the Earth/Moon pair causing the left to right tracking of the moon, shouldn't the lighted margins on both spheres have changed as it went by so that we were seeing progressively more dark side? Or does the extreme distance of the light source, the sun, make it not change in the view of the spacecraft? Also, are the continents of Australia and Asia visible as the video progresses?

[Wadsworth]

My 8th science class would to know...what phase of the moon would you say is visible at about 5 seconds of the clip. We are having a tough time with the hole rotation of Earth vs. revolution of the moon. Thanks!

It is a bit difficult to be certain, but the photos appear to have been taken when the moon is in it's 'Last Quarter' phase. See this illistration for reference:

https://www.space.com/62-earths-moon-ph ... aphic.html

A few things point to the last quarter phase, assuming the video is a progression of photos in the positive time direction. In the middle of the video (at about 5 sec) with the sun on the right and the rotation of earth as it is shown, the moon, the earth, and the camera on the satellite seem to more-or-less align. The shadow terminator lines on the earth and the moon look like they line up pretty well making this likely a 'Last Quarter' phase moon.

[MarkBour]

My 8th science class would to know...what phase of the moon would you say is visible at about 5 seconds of the clip. We are having a tough time with the hole rotation of Earth vs. revolution of the moon. Thanks!

I think it is great that your class is using this and thinking of a subtle question about it!

You can see from the rotation of features on Earth relative to the terminator, that during the period of the video, the Sun rose over much of the Pacific, Japan, Australia, and much of Asia/India. I would make a really crude estimate that about 8 hours elapsed from the first frame to the last (perhaps one of the links has more exact data on this). As Chris Peterson pointed out, then, the Moon did not go through very much of an orbit or change its phase as viewed from Earth much during that time. (If you watch a Moonrise one evening, then look at the Moon when it is 2/3 of the way across the sky -- about 8 hours later -- the change in phase is measurable, but very small.) During the video clip, for example, the Moon would have arisen from points of the Earth that rotated around the leftmost position visible on the Earth, so someone in Spain would have had a good evening of watching the Moon at the same time (if they had clear skies). And again, the phase of the Moon from different points of the Earth at a given time are also very very slightly different, but essentially the same.

Also, as heehaw pointed out, the apparent motion of the Moon versus the Earth here is almost completely due to the motion of the spacecraft. Like when you are riding in a car and watch some bushes near the road versus a building farther away. The Earth passed the Moon in an effect called parallax. The Moon is nearer to the spacecraft than Earth (as one of the links says, 6% nearer). If you watch closely, you'll see that the Moon's apparent motion in the video is almost perfectly a straight line, not a curve, which one would see if it was actually orbiting the Earth very much.

But, I'm taking quite a while to get at what I think you were asking: What was the phase of the Moon from the Earth that night?

I think you're trying to "eyeball it" from the video, to work it out yourselves. Given that they passed each other from parallax and that Earth is almost exactly at a quarter phase from the spacecraft, I would guess that the Moon is pretty nearly overhead of Earth's sunrise terminator at this point in time. So I would guess that the Moon was about at 3rd quarter phase, as did Wadsworth.

Of course the exact answer could be found by digging out an old calendar and looking at Dec. 16, 1992. Or, here: https://stardate.org/nightsky/moon

[De58te]

My 8th science class would to know...what phase of the moon would you say is visible at about 5 seconds of the clip. We are having a tough time with the hole rotation of Earth vs. revolution of the moon. Thanks!

First I'd need to determine whether the space probe took pictures with North up or South up. Some telescopes invert the image. Since we can see the clouds moving counterclockwise, we can figure that North is up. That means the Moon is in third or last quarter, and within 7 or 8 days will move between the Sun and Earth, New Moon.

[mko@malawicichlids.com]

"On rare occasions, the Earth and Moon are photographed together."
Actually, millions of people have done this. I've done it many times.
Oh, the **entire** Earth....

[bigtrain]

The moon is in the foreground. It revolves around the earth in roughly the same direction that the earth rotates. It is lunar phase 3rd quarter as viewed from the earth. North is up, as evidenced by which way the clouds are moving, and also you can see Australia appear at the bottom about 1/2 way through the video. Not so sure about the one posting that the moons motion is caused by movement of the spacecraft - you would think that the "phase" of the earth and moon would change if that were the case. I believe the moons motion in the video is a result of the natural revolution it is making around the earth. It appears as if the earth is getting a little smaller as the video progresses, so I think the slingshot flyby has already occurred and the spacecraft is receding from the earth/moon system.

[neufer]

Not so sure about the one posting that the moons motion is caused by movement of the spacecraft - you would think that the "phase" of the earth and moon would change if that were the case. I believe the moons motion in the video is a result of the natural revolution it is making around the earth. It appears as if the earth is getting a little smaller as the video progresses, so I think the slingshot flyby has already occurred and the spacecraft is receding from the earth/moon system.

• Indeed!

The video covers about half a day (~40,000 sec) based upon the Earth's rotation.

The Moon moves at about 1 km/s which is consistent with it's motion of ~5 Earth diameters (~40,000 km).

The distant spacecraft is moving much faster but almost radially.

[mfavret]

I'm french and I hope that you will excuse my poor English.

When I saw this movie yesterday, I was surprised by the strange Earth/Moon motion but I waited to read some comments about it. I did it this morning but I do not agree the spacecraft proper motion explanation.

If the spacecraft quick motion around Earth (so about the center of Earth) is revealed by the apparent motion of Moon, then Earth will have an apparent rotation of the same angle relatively to the camera but the "Earth's Terminator" is desesparately almost still, not going from a "Earth quater" to a "Earth Crescent". So the motion of Moon and Earth didn't seemed coherent for me.

In my first opinion, the author of the movie mounted a serie of images centered on Earth and because the Earth rotation made an artificial move of Moon that was quite still.

I captured the almost first and last pictures of this movie and edited it to put together the two earths and moons. It's obvious that there is a little change of their shapes (produced by the spacecraft motion) and there are consistant on the two objects. There is also a little rotation of the Moon because we see a crater appearing on the last image (with a 300% scale), probably Tycho. This rotation seems quite consistant with the estimation of 8 hours rotation estimated on the rotation of continents. In such a time, the rotation of Moon will be about 4° (period of 27 days), consistant with the apparition of Tycho near the terminator. In this time, the revolution of Moon around Earth is also about 4°(synchronous rotation) so about 30 000 km (based on a distance from Earth of 380 000 km), so about 17 times the moon radius. On the capured images, the Moon motion (about 791 pixels) is about 24 times the Moon radius (about 33 pixels).

So I must admit that I was wrong : the apparent Moon motion is mainly produced by its revolution around the Earth (about 17-18 radius or more if the movie duration is more than the estimated 8h) and probably the spacecraft motion (about 6-7 radius or less depanding on the real duration).

I think that it's a good illustration of how great are the planets/satellites speeds, largely sub-estimated in our first idea because of their slow motion when seen in the Earth sky !

Michel FAVRET

The composed image :

Click to view full size image

[blucat]

" Although our Moon may appear small next to the Earth, no other planet in our Solar System has a satellite so comparable in size."

Ganymede is the largest moon in the solar system, it's a moon of Jupiter, which is a planet. I know you all know this, so, what am I missing here?

[blucat]

Oh, I get it. Ambiguous wording. A moon so comparable in size to it's own planet, not so comparable in size to other moons in the solar system.