The Solar System is largely flat: Most of the planets, moons, and asteroids orbit near a common plane, known as the ecliptic. And yet, occasions when a nearer object hides a more distant one are uncommon.

Solar system bodies rarely line up perfectly because each one’s orbit has a unique tilt.

Though the major planets orbit within seven degrees of the ecliptic, they are so distant that the likelihood of two appearing in the same place at the same time is slim.

The moon’s orbital tilt, just over 5 degrees, carries it more than 10 times its apparent diameter off the ecliptic each month.

Today, the astronomical importance of lunar occultations of planets is minimal, with supercomputers to predict precise planetary positions.

Before the electronic age, these alignments provided valuable information about the local time, which in turn allowed calculation of one’s longitude. Lewis and Clark carried detailed almanacs of the moon’s position relative to bright stars and planets, which they used to navigate the uncharted West. Although they observed no lunar occultations of planets during their expedition, with modern hindsight we know that Lewis and Clark’s skills of observation and calculation allowed them to measure the distance they had traveled to an accuracy of 99% over their 4,000-mile trek to the Pacific Ocean.

Next Tuesday morning the moon will pass directly between earth and Mars for the first time since July 4 (only visible in Southeast Asia).

Idaho’s last lunar occultation of Mars was Nov. 13, 1998. Seeing Mars vanish behind the moon’s bright limb will be difficult, since it happens around 4:38 a.m. just minutes after moonrise (in the southeast). Reappearance, at 5:51 a.m., will be easier to see, with Mars popping out from behind the dark, upper right lunar limb.

Next column: To leap or not to leap.

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Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or canderson@csi.edu.


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