Doug Kerr
Well-known member
Last night, 2012.05.05 (in the US time zones), an astronomical event often popularly called a "supermoon" occurred. There are many misunderstandings about this event. (The actual time was 20120506 0335 UTC.)
The event was that a lunar perigee (the closest approach of the moon to the earth on some lunar orbital cycle) almost precisely coincided in time with the full moon.
Thus, we have a situation in which the visual size of the moon (its subtended angle) at full moon is greater than for other full moons (when the moon is not at perigee - is a little father away).
What made this particular event so "special" is that the times of the lunar perigee and of the full moon were so nearly identical - differing by about 2 minutes (I do not have the exact value at hand right now, and it hardly matters). (To the nearest minute, perigee was at 0334, full moon at 0336.)
This event did not represent the closet approach in many years of the moon to the earth, as we often hear stated. The perigee distance varies from lunar cycle to cycle, owing to the gravitational interaction between (mostly) the earth, moon, and sun. In 2011, for example, there were times at which the moon was closer to the earth than last night, but none of those times so closely coincided with the time of a full moon.
Basically, the subtended visual angle (apparent size) of the (entire) moon at perigee is about 6% greater at perigee than the average over a typical lunar cycle. When we heard that the moon was "14% bigger than normal", that referred to its subtended solid angle (its apparent area).
We heard that during last night's event the moon was "30% brighter than normal". The "inverse square law" is often cited in this connection.
We here realize that the perceived luminance of an an extended source (an object surface) does not vary with the distance to the observer (we assume no attenuation by the intervening medium). The inverse square law is not involved.
Thus, the luminance of the moon as seen from the earth does not depend on its distance from the earth.
It does depend on its solar illuminance - the degree to which it is illuminated by the sun. This varies with its distance from the sun, which of course varies slightly during a lunar cycle. (The inverse square law is involved there.) (There are also issues relating to the angles of incidence and observation and the non-lambertian nature of the moon's surface.)
This distance will be slightly less during a full moon that occurs at or near lunar perigee, but that is not of any great consequence, and the distance will vary to a greater degree over the year owing to the ellipticity of the earth's orbit around the sun.
The "supermoon" event is sometimes, but imprecisely, described "technically" as a lunar perigee-syzygy. Syzygy (in this context) refers to three astronomical bodies lying along a straight line. Since at full moon, the earth, sun, and moon nearly lie in a straight line, the term syzygy is often used by show-offs to mean "the situation of a full moon".
Of course, when the sun, moon, and earth actually lie precisely in a straight line (true syzygy), we have a lunar eclipse.
But not last night.
Best regards,
Doug
The event was that a lunar perigee (the closest approach of the moon to the earth on some lunar orbital cycle) almost precisely coincided in time with the full moon.
Thus, we have a situation in which the visual size of the moon (its subtended angle) at full moon is greater than for other full moons (when the moon is not at perigee - is a little father away).
What made this particular event so "special" is that the times of the lunar perigee and of the full moon were so nearly identical - differing by about 2 minutes (I do not have the exact value at hand right now, and it hardly matters). (To the nearest minute, perigee was at 0334, full moon at 0336.)
This event did not represent the closet approach in many years of the moon to the earth, as we often hear stated. The perigee distance varies from lunar cycle to cycle, owing to the gravitational interaction between (mostly) the earth, moon, and sun. In 2011, for example, there were times at which the moon was closer to the earth than last night, but none of those times so closely coincided with the time of a full moon.
Basically, the subtended visual angle (apparent size) of the (entire) moon at perigee is about 6% greater at perigee than the average over a typical lunar cycle. When we heard that the moon was "14% bigger than normal", that referred to its subtended solid angle (its apparent area).
We heard that during last night's event the moon was "30% brighter than normal". The "inverse square law" is often cited in this connection.
We here realize that the perceived luminance of an an extended source (an object surface) does not vary with the distance to the observer (we assume no attenuation by the intervening medium). The inverse square law is not involved.
Thus, the luminance of the moon as seen from the earth does not depend on its distance from the earth.
It does depend on its solar illuminance - the degree to which it is illuminated by the sun. This varies with its distance from the sun, which of course varies slightly during a lunar cycle. (The inverse square law is involved there.) (There are also issues relating to the angles of incidence and observation and the non-lambertian nature of the moon's surface.)
This distance will be slightly less during a full moon that occurs at or near lunar perigee, but that is not of any great consequence, and the distance will vary to a greater degree over the year owing to the ellipticity of the earth's orbit around the sun.
The "supermoon" event is sometimes, but imprecisely, described "technically" as a lunar perigee-syzygy. Syzygy (in this context) refers to three astronomical bodies lying along a straight line. Since at full moon, the earth, sun, and moon nearly lie in a straight line, the term syzygy is often used by show-offs to mean "the situation of a full moon".
Of course, when the sun, moon, and earth actually lie precisely in a straight line (true syzygy), we have a lunar eclipse.
But not last night.
Best regards,
Doug
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