Sky Watch

by J. Alex Knoll

Balance in the Heavens

Look to the spring moon to plot
the sun’s place in the fall

A gibbous moon wanes through late-evening and early-morning skies this week. Friday it crests the southeast horizon a little before 10pm, with the orange glow of Antares, the heart of the scorpion, trailing less than 10 degrees behind. The next evening, the moon rises almost an hour later, and now it lags behind Antares by about 10 degrees, the width of your fist extended at arm’s length. The two travel a shallow arc through southern skies, sinking toward the southwest horizon with dawn a few minutes before 6:00.

The moon’s path will dip even lower over the coming weeks until reaching its nadir around summer solstice. Six months hence, the autumnal sun will travel the same path during the day as the moon now travels at night, dipping lower and lower until winter solstice.

This juxtaposing of the sun and moon’s place is a result of earth’s 231⁄2-degree tilted axis, which also causes our changing seasons. At the two equinoxes, the path of the sun and moon, called the ecliptic, is parallel with earth’s axis. On these two days, the sun — and within a few days the moon — rises due east and sets due west. Over the rest of the year, the ecliptic climbs northward or recedes southward like a wobbling top.

With the return each spring of Antares and its parent constellation Scorpius, a hallmark of our winter skies disappears from view. See how many more nights you can spot the familiar hourglass shape of Orion the hunter low in the west-southwest. Without the glare of the moon, search for Orion’s sword, appearing as a dim fuzzy star midway between the three aligned stars of his belt and first-magnitude Rigel at his foot. A modest telescope or even strong binoculars will reveal this light for what it is — a celestial incubator of hundreds, even thousands of stars called the Orion nebula.