Monday, August 30, 2010

Moon image : scenic phenomenon

Full moon at Perigee and Apogee

From Perseus

A common misconception is that the moon is larger when it is near the horizon than when it is high overhead.
However, this optical illusion is not true, for the apparent size of the moon is virtually the same when it is rising or setting near the horizon or when viewed overhead (in fact, it is very slightly smaller when viewed near the horizon due to refraction as well as the greater added distance in observing across the earth's radius).
This illusion has been wrongly attributed to landmarks near the horizon, such as homes and trees, supposedly giving a sense of perspective and whereas the same perspective is lost when looking at the overhead moon bathed in an empty sky.
As noted by Donald E. Simanek and Carl J. Wenning, the real reason behind this trick by our brain is the perception of the moon being against a "close" or "distant" foreground and which is lucidly described by the above two references.

In contrast, it is puzzling that when a physical change in the apparent size of the moon does occur, due to its elliptical orbit around our planet, the change in the apparent diameter which can be up to 14% between apogee and perigee, is not noticed at all.
In fact, the change in the apparent diameter of the moon is a monthly phenomenon and is something that could be discerned quite easily during any given lunation by looking very carefully at the full moon and the waning crescent thirteen days later (or observing a waxing crescent thirteen days earlier)!

At apogee, the moon is approximately 406,500 km away from earth with an apparent diameter of about 29.5' whereas, at perigee, it is approximately 356,500 km away and is characterized with an apparent diameter of about 33.6'.
This difference of 50,000 km between apogee and perigee leads to the dramatic change in the apparent diameter as illustrated by the two full moons below which were strategically selected during 2010 so as to have the full moon as near to its minimum possible perigee and maximum possible apogee as possible when crossing the local meridian.

Furthermore, the apogee full moon below, captured during late summer, was slightly muted in colour when crossing the southern meridian due to its relatively low altitude during each summer and in contrast to the sun when the latter is at its highest during the same season.
These relative positions between the sun and the moon are juxtaposed six months later and during mid-winter with the (perigee) moon at a much higher altitude relative to six months earlier (note the absence of muted colouration due to atmospheric effects and sharper image) as well as relative to the sun.
The sun is now also much lower in the sky as compared to six months earlier (see here).

Note: The change in the apparent diameter of the sun due to perihelion and aphelion is fully documented elsewhere on this website (see here).

Links :
  • Wikipedia : Kepler's laws of planetary motion

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