For the next two weeks, Mercury, Venus, Mars, Jupiter and Saturn the five closest planets — should be easily visible at dusk, along with the moon.
"It's semi-unique," said Myles Standish, an astronomer at Jet Propulsion Laboratory in Pasadena, California. "They're all on the same side of the sun and stretched across the sky and that's what is kind of pretty."
Standish missed Monday evening's opportunity, but said Tuesday that he will gaze up when he walks his dog this week and next. He expects mountains and bright city lights to hamper his view, however.
The planetary lineup will be visible to the naked eye every night for an hour after sunset from around the world through early April. At the end of the year, the same five planets will reunite for a few weeks, but in the pre-dawn hours.
Standish said this particular planetary grouping may offer the best nighttime views until 2036.
The orbits of the five planets take them to the same side of the sun every few years or so. The conditions have to be just right for all five planets to be clearly visible at dusk or dawn; Mercury is often tough to catch. Even rarer are so-called alignments, where the planets are clustered together in the sky; this is not one of those.
Stargazers should look to the western horizon just after sunset. Mercury, Venus, Mars and Saturn will be lined up in the sky with Jupiter close to the eastern horizon. They will span about 135 degrees. Saturn will be almost directly overhead.