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Irishman
2006-Mar-20, 08:08 PM
At the Astronomy Picture of the Day (http://antwrp.gsfc.nasa.gov/apod/ap060320.html) page, they show artwork of a "SuperEarth" in orbit around a Red Dwarf star. Here is there description:


The above drawing gives an artist's depiction of what a super-Earth orbiting a distant red dwarf star might look like, complete with a hypothetical moon.

My comment concerns that hypothetical moon. That moon is entirely too close to the planet - it appears to be within the Roche limit. Tidal stresses would tear it to pieces. Note that in a similar scale picture, Earth's Moon would be located ~50 feet away. The conveyance of depth given in the picture doesn't allow to my mind the interpretation that we are just seeing a very convenient alignment that masks depth between the two objects. It's a bad artist's rendition.

01101001
2006-Mar-20, 08:31 PM
It doesn't seem all that different from, say, this picture of Dione and Saturn (http://photojournal.jpl.nasa.gov/catalog/PIA07749):

http://photojournal.jpl.nasa.gov/thumb/PIA07749.gif (http://photojournal.jpl.nasa.gov/catalog/PIA07749)

Irishman
2006-Mar-20, 09:33 PM
To me it does look different. I will try to describe how.

Dione fills the near view. We are obviously close to Dione. We know Saturn is HUGE, yet it is significantly smaller in the background, even though the rings are fairly large. Therefore, we can see the distance between the two bodies.

In the image I'm questioning, the moon is smaller than our field of view. The planet it orbits is much larger in our field of view. In order for there to be that amount of visual disparity and for there to be separation between the two bodies, that planet would have to be IMMENSE - like Jupiter-sized. But it is described as being 13 times mass Earth. So either we're looking at a pebble for the moon, or the two bodies are not substantially distant in depth. Ergo, the moon is within the Roche limit of the planet, and cannot form, and would be pulled apart by the tidal stresses. Unless we're witnessing a collision occurring, this picture isn't possible.

Irishman
2006-Mar-20, 09:45 PM
These representations are fairly common on these artist conceptions, and they're wrong. I think it comes from a common lack of grasping of the distance between planetary objects. For instance, ask people about the separation between the Earth and the Moon. If you give them a 2" diameter ball for the Moon. The Earth is about 3.67 Moons in diameter, so use an 8" ball for Earth - close enough for hand grenades. Ask them to place the balls with the proper scale distance. Most people will hold them up and try to put them a couple feet apart. Wrong. The Moon orbits Earth at a distance of about 30 Earth diameters, or at the scale we're using, the Moon needs to be 240 inches away, or 20 feet.

Wolverine
2006-Mar-21, 04:16 AM
Moved from Bad Astronomy Stories to Small Media at Large.

TriangleMan
2006-Mar-21, 12:09 PM
If in the APOD picture we assume that the planet has a mass 13 times that of Earth but the same density the radius would be around 2.35 times that of Earth (cube root of 13). So the Roche limit would be:

18,470 km x 2.35 = 43,405 km

Given that the radius of the hypothetical planet is around 14,800km (2.35 x earth radius) I can't see that moon being more than 40,000 km away so it does look like it is within the Roche limit. :think:

Gruesome
2006-Mar-21, 02:12 PM
I think it's all a matter of perspective. Below is a crude image I made of the Earth and the Moon. It's to scale (1mm = 1000km). I spun it around, attempting to match the perspective of the APOD picture and I think I got it pretty close.

The issue I have is the crescents of the APOD bodies. I tried putting the light [Sun] in a similar spot and they were barely visible. Nitpicking, I know, but still.

eburacum45
2006-Mar-21, 02:48 PM
I agree. The size of the bodies is a matter of distance, viewpoint and magnification; if you are far enough away from the planet you can 'flatten the image' by increasing the magnification. I do it all the time in Celestia; example here
http://img66.imageshack.us/img66/5474/zeus1jw.jpg
However from the viewpoint shown in the APOD image, the moon and the planet would both be very thin cresents.

Irishman
2006-Mar-21, 04:21 PM
Okay. It just looks misleading to me. I guess it's a perception thing.

Ara Pacis
2006-Mar-22, 04:54 AM
I don't see any problems. A photo like this could be taken at a suitable distance with a long enough lens to flatten the perspective. At astronomical distances like this you won't get depth of field cues with the objects in the photo. You've probably seen pictures of people "holding up" the Leaning Tower of Pisa... same idea. Although I suppose the visible stars might be bad astronomy.

Irishman
2006-Mar-27, 08:13 PM
I guess a valid question is are we trying to see what a telescope would show, or what the geometry is? 'Cause a telescope would show the full moon the size of a dinner plate, but it's only half a degree of arc in the night sky.

To me, I want to see it how it would look as an observer in some space ship (or force field bubble or whatever) local to the system, looking with "the unaided eye". YMMV.