APOD: Earth and Moon from MESSENGER (2010 Sep 01)

[Chris Peterson]

I hope this isn't too stupid a question, but why are there never phases of Earth from Mercury? It is because Mercury's orbit is so close to the sun? I tried figuring it out with a light bulb and a couple of balls, and got nowhere.

Earth does show phases as viewed from Mercury. But the range of phases you see on any superior planet is much smaller than seen on an inferior one. Inferior planets can go from nearly 100% illumination (when they are very close to being opposite the Sun from the observer), to 0% illumination (when they are between the Sun and the observer). This is exactly the way we see Venus and Mercury, as well as the Moon. Superior planets, however, never make such a large angle between the Sun and the observer, so they vary between 100% and somewhat less than that- but nowhere near 0%. If you think about the cycle of variation for a superior planet, it goes from 100% at opposition back to 100% at conjunction, with the minimum phase occurring near quadrature. Inferior planets don't have an opposition, so they go from 0% at inferior conjunction to 100% at superior conjunction, and back again.

Click to view full size image

[neufer]

If Earth was in Cancer from the viewpoint of MESSENGER, there's no way Aldebaran (in Taurus) could possibly be in the same image. It would be far out of the field of view... it is very unclear to me just what we are seeing here.

In fact, on May 6, 2010 it was MESSENGER that was in Taurus (below & to the right of the Pleiades) from the viewpoint of Earth according to NASA's Solar System Simulator

So the Earth (from MESSENGER's viewpoint) should really be near the ecliptic to the right in Scorpius (6 months away from Taurus)...

It looks like the angle between the Earth, Sun, and MESSENGER is about 12°, so that is almost one full month of difference, or one zodiacal constellation.

I'm not quite clear what you are arguing here.

Every object near the ecliptic observes every other object near the ecliptic in a position 180º along from where the other object observes it. And this is more or less means it is observed in the zodiacal sign 6 months away.

Note, however, that the earth almost observes MESSENGER in Aries so MESSENGER could well be observing the earth in Libra. This would make the likely star candidate Zubenelgenubi in Libra. Or it could be some fainter star.

When I said I didn't know what I was seeing here, I wasn't referring to the sky itself (although I don't, in fact, recognize the background), but to the nature of the image itself, which seems to show a scale that is not consistent with the instrument that is reported to have produced it.

Yes, that part is quite confusing.

(But you do agree that the questionable object is a star near the ecliptic don't you?)

[alter-ego]

This would make the likely star candidate Zubenelgenubi in Libra.

Bingo, α Librae.

[kimen]

i know the distance between earth and moon is more than 20 times the earth diameter. why in this picture the distance between earth and moon is almost equal the earth diameter?

[DavidLeodis]

I like the picture. It has both a simple yet awesome charm to it. The Earth and Moon also looks like a snowman built by a child (well OK, also by myself ) with a large round body and a small head.

[neufer]

i know the distance between earth and moon is more than 20 times the earth diameter. why in this picture the distance between earth and moon is almost equal the earth diameter?

Because they were in near conjunction that day (May 6, 2010) as observed by MESSENGER.

[Also because bright objects (such as the star on the right) appear larger in telescope mirrors than they actually are.]

[jerrythebiker]

Is it possible that the brighter object being discussed could be Andromeda? I am not clear on where in the constellations that Earth and the Moon appear, but I do think I recognize Perseus near the top center of the picture, just above and to the right of the Earth-Moon combo.

[bystander]

Is it possible that the brighter object being discussed could be Andromeda? I am not clear on where in the constellations that Earth and the Moon appear, but I do think I recognize Perseus near the top center of the picture, just above and to the right of the Earth-Moon combo.

The angular size of Andromeda would not be much different from Mercury (or MESSENGER)
than it is from Earth. That is to say, it would still be larger than the Moon as seen from Earth.

APOD: Moon Over Andromeda (2006 Dec 28)

[neufer]

Is it possible that the brighter object being discussed could be Andromeda?

No, the Andromeda galaxy is almost as big as the Earth as seen from the Moon.
It would be hundreds of times bigger than Earth as seen from 114 million miles away.

I am not clear on where in the constellations that Earth and the Moon appear, but I do think I recognize Perseus near the top center of the picture, just above and to the right of the Earth-Moon combo.

The field of view is too small to see a whole constellation.
We are (apparently) observing a small piece of a constellation within (or near) Aries or Scorpius.

[Chakolate]

Superior planets, however, never make such a large angle between the Sun and the observer, so they vary between 100% and somewhat less than that- but nowhere near 0%.

Okay, thanks. That's more like what I thought it should be - the original wording didn't quite cut it for me.

Your explanation was very clear - good job!

[alter-ego]

This would make the likely star candidate Zubenelgenubi in Libra.

Bingo, α Librae.

I was premature with this call. Nothing made sense when I tried to identify more stars, but I think I have now located the picture. It's in exactly the same location -- I made the wrong assumption about the APOD picture orientation (maybe most of us did). In the simulator, I zoomed in to the Earth (0.5º FOV) was surprised to find the Moon on the left side of the Earth not the right. The orbs are on the same axis, just reversed left to right! In fact it turns out the picture is clocked 180º from the simulator view (with a little added rotation). It's amazing how perspective can affect understanding.

Simultor view, 0.5º FOV
6 May 2010 0700 UTC
Earth - Moon axis marked

Below is a 20º FOV simulator view with the Earth - Moon, and the clocked APOD. The dotted lines show the Earth - Moon axis, and I've circled many stars that are common to both pictures. The particular star in question earlier is δ Scorpii, and the obvious pair of stars is ω1,2 Scorpii. I believe they are the same fields by virtue of how many common stars there are. I cannot find all the stars, but one field is a picture and the other is a simulation, so I'm happy and convinced they are the same.

Simulator view, 20º FOV
6 May 2010 0700 UTC
Earth - Moon axis transferred

APOD Clocked 180º - Messenger view of Earth/Moon, 10.5º FOV
6 May 2010 0700 UTC
Earth - Moon axis shown

That simulation works well - I wish we could enter a coordinate.
Bingo!

[Czerno-1]

Would the earth / moon pair be visible to the naked eye from this position? Or would one need binoculars or telescope?

No, if you were seeing this with your unaided eye, you would not be able to resolve the pair. .... at other points of the Moon's orbit you would easily be able to see the Earth and Moon as separate points of light. Neither would be big enough to resolve as a disc, however.

Hi Chris! The original poster's question was ambiguous. From that position, the Moon/earth pair could ordinarily well be separated by the naked eye, although of course not near to conjunction such as at the time the APOD was taken. My earlier answer has been incomplete in not stressing I had the max separation in mind, and numerous other respondents have explained the latter point. Thanks for making all clear again...

[neufer]

Nothing made sense when I tried to identify more stars, but I think I have now located the picture. It's in exactly the same location -- I made the wrong assumption about the APOD picture orientation (maybe most of us did). In the simulator, I zoomed in to the Earth (0.5º FOV) was surprised to find the Moon on the left side of the Earth not the right. The orbs are on the same axis, just reversed left to right! In fact it turns out the picture is clocked 180º from the simulator view (with a little added rotation). It's amazing how perspective can affect understanding.

The particular star in question earlier is δ Scorpii, and the obvious pair of stars is ω1,2 Scorpii. I believe they are the same fields by virtue of how many common stars there are. I cannot find all the stars, but one field is a picture and the other is a simulation, so I'm happy and convinced they are the same.

Excellent piece of sleuthing, alter-ego

Dschubba : δ Scorpii...so I wasn't so far off with my original guess.

<<Delta Scorpii (δ Sco / δ Scorpii) is a star in the constellation Scorpius. It has the traditional name Dschubba (or Dzuba, from Arabic jabhat, "forehead" (of the scorpion) or also Iclarcrau or Iclarkrav. Because Delta Scorpii is near the ecliptic it is occasionally occulted by the Moon, or (extremely rare) by planets.

In June 2000, Delta Scorpii was observed by Sebastian Otero to be 0.1 magnitudes brighter than normal. Its brightness has varied since then and has reached as high as magnitude 1.6 or 1.7, altering the familiar appearance of Scorpius. Spectra taken after the outburst began have shown that Delta Sco is throwing off luminous gases from its equatorial region. As of 2005 the flareup continues. Although the brightness varies, it remains well above its previous constant magnitude.

Dschubba is accompanied by a class B star that orbits the primary every 20 days at a distance comparable to the distance from the Sun to Mercury. Furthermore, there is a star that takes about 10 years to orbit Dschubba in a highly eccentric orbit that takes it close in to the primary once a decade. The last close encounter of these two stars happened in mid-2000 and it may have triggered the outburst of the primary star. A possible fourth companion star lies at about twice the distance again from the main star.>>

<<The Bayer designation Omega Scorpii (ω Sco / ω Scorpii) is shared by two stars, ω¹ Scorpii and ω² Scorpii, in the constellation Scorpius. They are separated by 0.24° on the sky. [Ergo: The ~10° size of the APOD picture is about right] Omega Scorpii also has the traditional name Jabhat al Akrab, which is derived from the Arabic جبهة العقرب jabhat al-caqrab meaning "[The] forehead of the scorpion".

[Double star above: Beta Scorpii in http://antwrp.gsfc.nasa.gov/apod/ap090521.html. ]

ω¹ Scorpii : Omega-1 Scorpii is a blue-white B-type dwarf with an apparent magnitude of +3.93. It is approximately 424 light years from Earth.
ω² Scorpii : Omega-2 Scorpii is a yellow G-type bright giant with an apparent magnitude of +4.31. It is approximately 265 light years from Earth.>>

[neufer]

i didn't vote for this image and regret that it was posted because it grossly misrepresents both the relative sizes of the earth and moon (because the moon has a higher albedo) and grossly misrepresents the earth/moon distance relative to size.

image size of earth = 11 pixels
image size of moon = 5 pixels
actual earth diameter = 7926 miles
therefore, actual moon diameter = 7926 * 5/11 = 3600 miles OH SO WRONG!

earth based pixel scale = 7926/11 = 720 miles
image separation earth/moon = 12 pixels
therefore, earth/moon orbital distance = 720*12 = 8647 miles OH SO WRONG!

i've attached an image below that shows the correct proportions without image flare and/or the conjectural, post hoc suggestion of orbital foreshortening.

when you have viewers who dwell on the flattening of the earth on one side, without noticing that all the star images are elongated vertically, you realize the importance of posting astronomical images that do not invite gross misinterpretation, especially when the picture commentary makes no attempt to correct them.

What it shows is their relative brightness at the distance specified. As for the separation problem, the Moon was more than likely not at its maximum apparent distance from the Earth. It could have been in front of the Earth relative to the camera position thereby making it appear both too close and too bright.

Earth Moon on May 6, 2010

---

In fact, the Moon was at its maximum apparent
distance from the Earth at this time =>

"drollere" & "alter-ego" were right about the misrepresentation that the APOD picture gives.

[nealmcb]

Good job, alter-ego! I came to the same conclusions, and got coordinates from the JPL Horizons system (http://ssd.jpl.nasa.gov/?horizons). The time of the picture is more like 8:00 UTC on May 6, and Horizons says the earth was at R.A. 15 37 27.02 Dec -20 47 26.1 and distance 1.22525823308655 AU then. I also linked to a more detailed starfield from skyview. Here is my blog post about it:

http://neal.mcburnett.org/blog/2010/09/ ... et-so-far/

Neal McBurnett

[Chris Peterson]

In fact, the Moon was at its maximum apparent
distance from the Earth at this time =>

Which solves the apparent confusion with image scale. The spacing between the Earth and the Moon in the image is exactly as you'd expect for a 1024x1024 pixel camera with a 10.5° field of view. What this means is that neither the Earth nor the Moon is even close to being resolved in the image. Each is actually much smaller than a single pixel. The apparent sizes are purely a function of their brightness and the combination of diffraction and optical artifacts. The relative difference in size is determined by their relative difference in brightness. The only thing actually resolved in the image is the separation between the two bodies.

[nealmcb]

The moon, by the way, is at 15 37 58.45 -20 48 46.0 and 1.22571799940636 AU, so about 7.5 minutes of arc separation from the earth - easy visual-eye, it would seem. If you ask Horizons for the Moon's coordinates, it gives you things like the angle between the Moon and Messenger, as seen from the Moon's primary (the Earth): 99.7 degrees in this case, which fits with it being a bit farther away. Horizons also gives lots of other angles - it is so wonderful!

[alter-ego]

Which solves the apparent confusion with image scale. The spacing between the Earth and the Moon in the image is exactly as you'd expect for a 1024x1024 pixel camera with a 10.5° field of view. What this means is that neither the Earth nor the Moon is even close to being resolved in the image. Each is actually much smaller than a single pixel. The apparent sizes are purely a function of their brightness and the combination of diffraction and optical artifacts. The relative difference in size is determined by their relative difference in brightness. The only thing actually resolved in the image is the separation between the two bodies.

This scale factor business was an interesting diversion to me since I wasn't really aware of it - I didn't endeavor to pursue pixel discussion details . The simplest and very first exercise I did was verify the scale by carefully measuring (with a mm ruler) the earth/moon centroid separation and the picture size on my monitor. Ratioing the two dimensions and using 240,000 miles as the sparation, I calculated an image FOV = 10.1°. I immediately concluded the stated FOV was close enough to set my search scale wrt the simulator. I therefore was never inclined to believe the image sizes of the earth and moon. I would have mentioned the simple measurement test, but I immediately got engrossed in the harder problem of locating the image field location which I would have floundered with if I didn't believe the picture FOV.

Excellent piece of sleuthing, alter-ego

Thank you, Art. Sleuthing...it's one of my favorite pass-times.
ae

[odysseus14]

I was just wondering what that object is, on the same horizontal line, as the earth and moon, off to the right of the image. Is that Jupiter off in the distance?

[bystander]

I was just wondering what that object is, on the same horizontal line, as the earth and moon, off to the right of the image. Is that Jupiter off in the distance?

It's the star δ Scorpii. See:
http://asterisk.apod.com/vie ... 17#p131217