Subsolar point

Subsolar Point

(glossary entry)

Description

The subsolar point is the location on the surface of the Moon where Sun is directly overhead. It can be found by drawing an imaginary line from the center of the Moon to the center of the Sun, and is normally described by giving the selenographic longitude and latitude of that point. Because the Moon's spin axis is tilted only 1.5° to the plane containing the Sun, the subsolar point can never wander more than 1.5° from the Moon's equator. However it can, and, does, move through all longitudes. This motion is westward (in the direction of decreasing selenographic longitude), and at a rate of about 12.2°/day. - Jim Mosher
 
 

Additional Information

• The term solar colongitude is also widely used in the lunar literature to describe the location of the subsolar point. The colongitude is 90° minus the selenographic longitude (measured with eastward positive) of the subsolar point. Because of the subtraction, the colongitude of the subsolar point increases with time. - Jim Mosher
• There is an interesting phenomenon associated with the subsolar point which was (and is) frequently depicted by various Space Artists and Illustrators (Chesley Bonestell, William K. Hartmann, etc...). During high sun, the moon's surface is a featureless "snowy white" surface without shadows. There's only the undulating appearance of distant hills and hillocks. They too look shadowless and without "depth". During the last three Apollo missions (the scientific "J-missions" of Apollo 15, 16, and 17) the concluding LEVAs (Lunar ExtraVehicular Activities) were always performed during high sun (the "before noon" sun). It was rather difficult to detect craters and other uneven obstacles (only the shadows of rocks and boulders were seen). The Hasselblad photographs and TV-transmissions which were made during these concluding LEVAs (and at the so-called "VIP-site") show featureless surfaces and "white" mountains, almost as if the astronauts explored a Terrestrial desert, or Antarctic snow... (related: the frightful "White Out" phenomenon of Arctic explorers). - DannyCaes Feb 10, 2008
• A typical look at a "shadowless" lunar surface is Mike Constantine's assembled panorama of Apollo 15's Station 9a (at the "Terrace") near Rima Hadley. No shadows of craters and craterlets are visible, only the "black spots" of the rocks and boulders. At left (near the panorama's left margin) is Mons Hadley. At centre is the Swann Range with Big Rock Mountain (one of the two peaks). At right is Rima Hadley's "Trophy Point " (the inner curve of the rille's "elbow"), and behind it ("above it") the St. George crater on Mons Hadley Delta's northern slope. - DannyCaes Feb 10, 2008
• Another look at a "shadowless" lunar surface is Dave Byrne's assembled panorama of Apollo 15's VIP-site (the final parking place of the Lunar Roving Vehicle).
• Please note that because the Apollo 15 Site is 26° from the Moon's equator, it is always at least 24.5° from the subsolar point. Hence the seemingly shadowless views at this site mentioned by Danny are not technically the result of the astronauts being at the subsolar point. The visibility or non-visibility of shadows is, in general, a combination of the illumination and viewing angles. A high Sun certainly shortens shadows, and eliminates them for smooth surfaces tilted by less than the angle of the sunlight. But at any sun angle it is possible to completely hide the shadows by looking along the direction of the sunlight (leading to the so-called opposition surge). On the other hand, it is also possible to find shadows at any sun angle simply by looking in the right direction. For example, even exactly at the subsolar point (with the Sun directly overhead) any overhanging structure (such as a spacecraft raised on struts, or an undercut boulder) will cast shadows directly downwards. These shadows cannot be seen from above (looking along the direction of the sunlight -- because they are blocked from view by the shadow-casting object), but they can be seen from the side. The primary reason for an expected difference between the appearance of the lunar landscape at the subsolar point versus an equatorial terrestrial landscape (with the Sun similarly directly overhead) is the smaller number of overhanging features on the Moon. Any similarity of lunar views seen by astronauts to phenomena observed by Arctic explorers on Earth would seem more to have to do with looking along the direction of the sunlight (rather than with the Sun being high), since, like the Moon's poles, the Earth's poles are always far from the subsolar point and illuminated at a very low Sun angle. - Jim Mosher

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