Colorado Skies: An inferior conjunction that's not so inferior

(Editor’s Note: This is the first appearance in the North Forty News of a long-standing monthly column on popular astronomy titled “Colorado Skies.” The series is written by Roger B. Culver, who has taught astronomy at Colorado State University for 50 years.)

In the orbital hierarchy of the solar system, two planets, Mercury and Venus, travel in orbital paths that are closer to the sun than that of the Earth. Because these planets are closer to the sun, they orbit the sun more rapidly than the Earth, regularly passing between the Earth, and the sun. When such an “in between” passage occurs, it is referred to by astronomers as an inferior conjunction.

Because the orbits of Mercury and Venus are at slight angles to the planet of the Earth’s orbit, these planets are usually located north or south of the earth’s orbital plane at the time of an inferior conjunction. Twice during a given orbital cycle, however, Mercury and Venus pass through the Earth’s orbital plane at points that are called the nodal points or nodes. On relatively rare occasions it is possible for Mercury or Venus to be located exactly at a nodal point at the exact time of an inferior conjunction. The result is a most interesting astronomical event called a transit.

Transits can be thought of as a sort of “mini-eclipse” in which either Mercury or Venus valiantly attempts to blot out the sun’s visible disk in a fashion similar to what the moon is able to accomplish during a total eclipse of the sun. Unfortunately, even though Mercury and Venus are larger than the moon in linear size, they are much farther away and the net result seen from the Earth is a small dark circular disk transiting the face of a much larger angular diameter sun.

By precisely timing when the planetary disk enters and exits the sun’s background edge, astronomers can gain valuable information regarding the sizes and shapes of both the sun and the planet, the distance to the sun, and any long-term, small-scale changes in the orientation of the planet’s orbit. It was through just such precise measurements made during the 18th and 19th centuries on the planet Mercury that astronomers were able to determine that Mercury’s orbit was changing its orientation at a nearly imperceptible, but nonetheless detectable rate.

For over a century these mysterious orbital movements could not be explained by scientists. In 1916, however, Albert Einstein’s Theory of General Relativity, with its attendant space-time warps, triumphantly succeeded where all previous theories had failed.

Because of the strict constraints on planetary positioning, transits of Mercury and Venus are relatively rare celestial events. Only 14 transits of Mercury occurred during the 20th century and the transit of Venus in 2004 was the first such event since 1882.

With these facts in mind, it is noteworthy that on the morning of May 9 Fort Collins residents will have their first opportunity since 2006 to view one of these transit events in this month’s Colorado sky when the planet Mercury will transit the sun’s disk for over seven hours.

Starting around 5:12 a.m. MDT the tiny dark disk of Mercury will enter that of the sun, and will, for over seven hours, creep almost directly across the center of the sun’s disk before exiting around 12:45 p.m. This event can be safely observed by using a small telescope to project the sun’s image onto some sort of poster board screen. Please be reminded in no uncertain terms that the greatest care should always be exercised in observing the sun’s image, either with or without optical aid.

Elsewhere in the sky: At the onset of darkness the planet Jupiter shines almost directly overhead as a prominent yellowish-white object just to the south of the constellation of Leo, and can be readily seen throughout the first half of the night.

On the evening of May 22 the planet Mars reaches opposition to the sun. On that night Mars will rise at sunset, set at sunrise, and will be visible all night long. The Red Planet will be closer to the Earth than at any time in over a decade and is easily viewed as a sort of bright “celestial garnet” in western Scorpius.

The planet Saturn rises about a half hour after Mars and forms a nice triangular configuration with Mars and the bright ruddy Scorpius star Antares.

The planet Venus is too close to the sun to be easily seen in May.

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