Explained: Apollo 17 Photo of Earth from Moon Seems too High

drhex

New Member
This photo http://www.nasa.gov/sites/default/files/thumbnails/image/as17-134-20387.jpg which shows an astronaut on the moon with the Earth visible in the sky, has come up in a facebook discussion:
as17-134-20387.jpg


https://www.facebook.com/groups/169...921661&notif_t=like&notif_id=1474296596529377
Content from External Source

The claim is that Earth appears too low in the sky for image to be real.
What I've found out so far: The link is to a Nasa website and so should not be altered by anyone else.
Googling the filename show that is is from Apollo 17 and thus Taurus-Littrow.
Using astronomy software "stellarium", I found that Earth should be 44-60 degrees above the horizon from Taurus-Littrow, and to be more precise at 45 degrees on december 11-12 1972 during Apollo 17.

I'm no expert on image analysis but Earth should use up about 2 degrees of the field of vision when seen from the moon. Counting how many Earths fit between its position in the image and the horizon, it appears to be about 25 degrees up here, which is indeed too low.
 
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Mick West

Administrator
Staff member
Using astronomy software "stellarium", I found that Earth should be 44-60 degrees above the horizon from Taurus-Littrow, and to be more precise at 45 degrees on december 11-12 1972 during Apollo 17.
Can you show your working?
 

Nada Truther

Active Member
The middle pic in the top row is clearly taken from a low angle. Probably to get the earth in the shot. Because, it probably was at a 45 degree angle from the horizon and wouldn't be in the shot. Some good photography skills recognizing how good the shot of old glory would look with Terra Firma in the background. The actually shot is less obvious to tell if it is taken from a low angle, but the horizon is hardly a horizon. Unless it is a fish-eye lens, it would most likely be a hill or some other steep land formation, as previously stated. This just goes to show how one person can view something and be sure of one outcome, and another can look at it from a completely view-point angle and get a completely different result. The proof is in the research. One image would yield less variation, but multiple images will show much more information to arrive at the conclusion that it is a hill, not the horizon.
 

Hama Neggs

Senior Member.
It should be obvious that the very curved surface of land is not a normal Lunar horizon. Amazing how people can overlook the obvious when declaring that they have obvious proof of something.
 

Trailblazer

Moderator
Staff member
The hill is the South Massif. Based on this topographic map, the South Massif summit is at 7141 metres, while the landing site is at about 4680 metres, giving a height difference of about 2460 metres.

upload_2016-9-20_18-42-43.png

Measuring on Photoshop using the ruler tool gives a distance of almost exactly 12 kilometres from the landing site to the summit. Ignoring the moon's curvature, that would give an altitude above the horizon of arctan (2460/12000) = 11.6 degrees.

It's hard to measure the exact height from the photo in the OP as the top of the hill is obscured.
 

Trailblazer

Moderator
Staff member
This photo gives a better idea.




Rotated:

upload_2016-9-20_19-5-36.png

Measuring that in Photoshop I get an angle of about 15 Earth diameters, or 29 degrees based on an Earth angular diameter of 1.9 degrees, from the centre of the Earth to the top of the hill. If we then add the 11.6 degree height of the hill we get to 40.6 degrees, which is not all that far from what it should be, given the uncertainties in measurement.
 

Trailblazer

Moderator
Staff member
I haven't checked the actual elevation of the Earth during Apollo 17, but I found this discussion on the Collectspace forums, where posters generally know their stuff:

From all the Apollo missions landing on the moon, Apollo 17 had the Earth at the lowest altitude above the horizon perpendicular to the local vertical at the landing site.

Not only the latitude is a factor but the longitude of the landing site as well, since the near side of the Moon always more or less (libration) faces the Earth. The Taurus-Littrow valley is not only located 20º north of the lunar equator but also 30º east of the null meridian (that is to the right of the center vertical 0º longitude when you look at the Moon from Earth), therefore the Earth was approximately only 42º above the local horizon. Also, due to the longitude of this landing site (and the given sun angles at landing time constraints) the crew of Apollo 17 saw the Earth at the highest disk illumination during the lunar surface stay of all missions.

One photo showing Gene and the Earth above is this one: http://www.hq.nasa.gov/office/pao/History/alsj/a17/AS17-134-20387.jpg

During EVA 2 at Station 2 at Nansen crater, they took a photo "upslope" the South Massif with the Earth in the image as well. This photo looks like the Earth would be low over the horizon but of course this is only due to the 30º slope of the massif at the foot they are standing of. The respective photo is this here: http://www.hq.nasa.gov/office/pao/History/alsj/a17/AS17-137-20957.jpg
Content from External Source
http://www.collectspace.com/ubb/Forum29/HTML/000839.html
 

Mick West

Administrator
Staff member
On Google Earth (Moon) the top of that hill is about 11° above level from the lander site.. But I'm not sure how accurate GE is for that type of thing. KMZ attached
20160920-113956-d13rf.jpg
 

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I've done an extensive study of All Apollo's Earth images, and I've looked at that one on this my site - this image is from there and shows the visible and infra red images of Earth on the day the Apollo image was taken:




I've also looked at some images from Apollo where Earth is visible in reflected in astronaut visors, including Apollo 17.

Here's one of the visor shots:



and here's what Stellarium says should be visible:



Earth is exactly where it should be in the Apollo images.

I'd recommend getting Stellarium, setting the location for APollo 17 and the time for during the mission and you'll see where Earth should be and what it should look like :)

Anyone who doubts how big the Earth is in Apollo images from the moon needs only to go outside and take a picture of the moon, then compare what they see on their photo with what they can see in the sky.
 
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drhex

New Member
It should be obvious that the very curved surface of land is not a normal Lunar horizon. Amazing how people can overlook the obvious when declaring that they have obvious proof of something.
I also thought the surface looked too curved, but assumed that was due to wide angle lens distortion. With more context, that South Massif is a better explanation. Thanks everyone.
 

Redwood

Active Member
Another minor point is that although the Earth, as viewed from the Moon, stays "pretty much" in the same position in the sky, it does vary.

The moon's orbit is tilted 5 deg. to the ecliptic, and its axis is tilted another 1.5 deg. From the Moon's poles, the Earth will actually appear to slowly rise and set, with the maximum setting occurring (for the north pole) when the moon is at its northernmost point in its orbit relative to Earth, and the Moon's north pole is tilted at maximum away from the Earth. Vice-versa for the South Pole. At Moon latitudes in between, the effect is less but is still measurable.
 

Trailblazer

Moderator
Staff member
You lost me. What axis is tilted to the Moon's poles? Earth's?
The axis of the moon's rotation is tilted 1.5 degrees from the plane of the moon's orbit. (For comparison, the Earth's axis has a tilt of 23.5 degrees relative to the Earth's orbit.)

This picture shows the tilts relative to their own orbital planes, not relative to each other.

 

Hama Neggs

Senior Member.
The axis of the moon's rotation is tilted 1.5 degrees from the plane of the moon's orbit. (For comparison, the Earth's axis has a tilt of 23.5 degrees relative to the Earth's orbit.)

This picture shows the tilts relative to their own orbital planes, not relative to each other.


Oh... right. I read the other post wrong.
 

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