The Skies, Explained

Three kinds of sky events fill our Noteworthy Skies list. Here's what each one actually is. Don't worry — the truth doesn't drain any of the wonder out. If anything, it's better.

A retrograde is an illusion of perspective, and it's the planets' most famous trick. Planets don't stop, and they don't back up, and they don't misbehave. Mercury, which orbits inside Earth's path, laps us three or four times a year — and as it swings around, it looks, from our own moving viewpoint, like it slows down and loops backward against the stars. It's the same thing that happens when a car you pass on the highway seems to slide backward against the mountains. The outer planets do it when Earth, on her faster inside track, overtakes them. Astrology made retrogrades famous as seasons for reviewing and returning. Astronomy says it's geometry. In this Grove, you're free to enjoy both.

An eclipse is an alignment — the technical word is syzygy, three bodies in a row. A solar eclipse happens when the new moon passes exactly between the Earth and the sun. A lunar eclipse is the full moon crossing through Earth's shadow, and a penumbral eclipse, the subtlest kind, just brushes the shadow's edge and dims the moon a shade. Eclipses come in seasons, about every six months, when the moon's tilted orbit crosses the sun's path.

And a meteor shower is Earth crossing the dusty trail of a comet. Comets shed grit as they travel, and when our orbit passes through the stream, those grains burn up in our air. A "shooting star" is a speck of dust, dying beautifully. Each shower is named for the constellation it seems to pour out of — the Geminids from Gemini, the Lyrids from Lyra. The Geminids are the rebels of the family: their parent isn't a comet at all, but an asteroid called 3200 Phaethon.

In the Grove: we list them so you'll be outside on the right night, looking up. Idaho's dark skies will do the rest.