earth phases

[just_curious]

Would the earth if viewed from the moon go through phases like the moon and if so would the phase process take the same amount of time?<br /><br />Also, would the earth always be the same size?

 

[vogon13]

Yes and yes.<br /><br />The apparent size of the earth as seen from the moon would very slightly in sync with the ever changing earth moon distance. Horizon to zenith size change effect (actual measured change, not 'horizon illusion') seen from earth of moon does not occur from moon viewer looking at earth since moon is tide locked to earth. (for simplicity, have neglected effect of libration, which wouldn't make that big of a change anyhow). <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[thalion]

Basically to echo what Vogon wrote:<br /><br />1.) Yes.<br /><br />2.) No; the Earth would vary slightly in size with Earth's distance. Starry Night indicates that it would also change in position very slightly, due probably to its regular slow orbital cycles (changing inclination, orbital orientation, ecccentricity), but this would probably only be detectable with instruments over a long period of time.

 

[just_curious]

In the two images from Apollo 11:<br /><br />AS 11-44-6547 and AS 11-44-6552<br /><br />AS11-44-6547<br />View of the Earthrise from the Command Module Columbia. This picture was taken shortly after Earthrise as Columbia was passing over Mare Smythii. In looking at Earth, Australia is at the left, just above the lunar horizon. 20 July 1969. Scan by Kipp Teague.<br /><br /><br />AS11-44-6642 <br />Eagle is approaching the Command Module during rendezvous. The LM hatch is pointed toward us and the Ascent Engine is pointed up and to the left. Scans by Kipp Teague.<br /><br /><br />The earth is very large in the 6552 image compared to the 6642. Is there a reason for this?<br /><br />http://history.nasa.gov/alsj/a11/images11.html

 

[CalliArcale]

Framing of the picture probably makes a difference -- those images have been post-processed and cropped to make them look nice. They may also have brought along a telephoto lens for all I know, which would also change it. It's hard to judge relative size in photographs without a frame of reference (i.e. something close to the same distance to which it can be compared, to eliminate the problem of false perspective). <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[vogon13]

And a related topic you didn't ask about, for a suitably distant observer of both the earth and the moon, both would show the same phase at the same time. One of the Mars orbiting spacecraft took a picture of 'the home world' a while back that showed this. Also Cassini, I believe, did a time lapse of the moon passing across field of view showing earth and it also verified this. Very neat pictures, and a neat way of looking at phases. <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[CalliArcale]

I've got a little collection of some of those pictures on my hard drive, because they are so beautiful. Most are post-processed so you can see both Earth and the Moon; the Moon is considerably darker than the Earth. Here's one taken by Voyager 1 as it receded from the Earth, just a few days after launch. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>