Quote:
Originally Posted by michaellxv
Firstly, love the images.
On the question of shape, I thought that the outer planets did not show any phase (to us) and that we would always see them fully illuminated.
Is this different for Mars due to its relatively close proximity? Or am I missing something?
How far do we see this phase go before we loose Mars to daylight?
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Hi Michaell,
It is true that only the "inferior" planets, that is the planets inside the orbit of the Earth exhibit relatively extreme phases such as we see with the Moon.
Consider the case with the Moon, in particular, consider the geometry of the Sun , Earth, Moon at full Moon and a couple of days either side of Full Moon.
At full Moon the Moon is indeed at a greater radii from the Sun than is the Earth, it is opposite the Earth relative to the Sun and we see it to be round with the full disk illuminated. However, a couple of days either side of full, while it is still at a greater radii from the Sun than is the Earth, we no longer see the full disk illuminated, we see a phase of the full disk.
When the planets are imaged at very high magnification, even slight variations in the shape of the apparent disk are noticeable, as you have noted, Mars is relatively close to us and so the changes in the Sun, Earth, Mars angles are enough to reveal this slight change in phase.
With accurate enough equipment this effect would likely be possible to measure with the other planets, but it would be very small. If you have access to "The Sky" or "Starry Night" and bring up one of the planets and view the data on it for any specific date, it will provide the phase, if you then advance the date by a couple of months you will see from the data for that planet that the phase has changed.
Regards
Trevor