Astro Bob blog: In praise of bicuspidsCurves and cusps across the solar system
By: Bob King, Duluth News Tribune
In praise of bicuspids
The very thin crescent moon will be visible low in the eastern sky about 40 minutes before sunrise tomorrow morning. The map shows the approximate position of the sun, which is below the horizon at the time, to help you gauge the separation between it and the moon. Created with Stellarium
The moon is headed toward New Moon phase and a fine conjunction with Venus early next week but before it does those with a wide open view to the east can spot the skinny waning crescent tomorrow morning before sunrise. If you like the look of a crescent moon you may want to explore other moon crescents elsewhere in the solar system. Venus and Mercury sadly have no moons but the rest of the planets all come with company. Here are the current moon totals:
* Earth: 1
* Mars: 2
* Jupiter: 63
* Saturn: 62
* Uranus: 27
* Neptune: 13
* Pluto: 3
Add 'em all up and you get 171. Thanks to spacecraft flybys and orbiters we've been able to see many of them up close and showing phases just like our own moon. The key to seeing moons or even planets as crescents is to get around their backsides so they're nearly lined up with the sun. From this perspective we see only their edge or rim illuminated by the sun, and an "edge" on a spherical body is naturally a crescent of light. Spacecraft can do this for many different solar system bodies because we send them to faraway places like the backside of Jupiter or behind Saturn's moon Titan. You and I meanwhile are literally stuck on Earth -- with the exception of Mercury and Venus.
Using Venus as an example, we see how planets and moons look like crescents (C and D) when they're nearly in line with Earth and the sun. From our perspective behind Venus we see mostly the planet's backside with a thin sunlit edge. Technically, a spherical planet or moon will look like a crescent if the angle between it, the sun and Earth is greater than zero but less than 90 degrees. Illustration: Bob King
We see Mercury and Venus as crescents during part of their revolutionary cycles because Earth is in the next orbit out. When the two planets get approximately between us and the sun we see their backsides. If you've never seen them in this phase through a telescope, you'd be surprised how much they mimic the lunar crescent. When the planet-sun-Earth angle is exactly 90 degrees, the planet looks half lit as in B and E above. Less than 90 and it starts to assume a crescent phase, fat at first and then thinning as the angle between the three narrows.
How about double the fun? In this sequence of images taken by the Cassini spacecraft, Saturn's moon Enceladus (foreground) passes in front of larger Rhea. Credit: NASA/JPL/Space Science Institute
There's beauty in that hammock-like arc contrasting with the sharpness of the cusps. For your pleasure, I've selected several alien crescents for your perusal. All these images come to you courtesy of our robotic emissaries who've gone the distance and turned around to look back from perspectives not available to humans ... yet.
This photo of Jupiter's moon Io was taken on March 2, 2007 by the New Horizons spacecraft that flew by Jupiter en route to Pluto. Backlighting from sunlight brought three active volcanic plumes into clear view including the 190-mile high plume from the Tvashtar volcano at top. It appears blue because of the scattering of light by tiny dust particles ejected by the volcano, similar to the blue appearance of smoke. You can see the red glow of lava at the plume's source. The right side of Io is faintly lit by sunlight reflecting off Jupiter. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
The planet Neptune and largest moon Triton were photographed by the Voyager 2 spacecraft during its flyby in 1989. Credit: NASA
Saturn as seen by Cassini in 2006. The weird shadows are cast by the rings which extend down toward the bottom of the picture. Credit: NASA/JPL/Space Science Institute
OK, so it's not exactly a crescent but you get the idea. Saturn's 20-mile diameter moon Helene is irregular in shape so its "crescent" is too. Helene's backside is dimly illuminated by sunlight reflecting off Saturn. The picture was taken during Cassini's close flyby on March 3. Credit: NASA/JPL/Space Science Institute