To twinkle or not to twinkle, that is the question

Venus passes Regulus on the morning of September 5. Look low in the eastern sky 30-45 minutes before sunrise to see the pair. Bring binoculars in case twilight overwhelms Regulus. Stellarium

Early Friday morning September 5, skywatchers will see Venus and Leo’s brightest star Regulus in a close conjunction. The two will be separated by just 1° and look very nice in binoculars. Find a place with a view down to the eastern horizon and start looking about 40 minutes before sunrise. Jupiter, higher up in a darker sky, can help guide you to Venus.

This will be Venus’ last encounter with a bright star at dawn before it’s lost in the glare of the sun. It’s often said that one way you can tell a planet from a star is that a planet’s light appears steady, while stars twinkle. Not always. Stars only appear as points of light even through the largest telescopes and are easily shoved this way and that by air turbulence. These tiny shifts in position are what cause twinkling.

When we look at stars low in the sky we look across hundreds of miles of air in the lower, densest part of the atmosphere. Air currents across that great distance push a star’s light around causing it to twinkle. It can have the same effect on bright, naked eye planets when they’re far away and show a smaller than usual disk. Credit: Bob King

Planets have measurable disks and are less affected by the flutter of air, so we rarely catch them shimmering. But when the planet is far from Earth and very low in the sky, the rules change. Both Venus and Mars range in size from tiny blips to substantial disks (or in the case of Venus, a substantial half-moon or crescent). When viewed at low altitude, I’ve seen both twinkle lively.

Illustration showing how a planet, with a measurable disk, defeats air turbulence compared to a star which appears as a tiny point of light through a telescope. Credit: Bob King with Jupiter pic by Damian Peach

I witnessed it last Thursday morning with Venus. Jupiter, larger and higher in the sky, was a steady beacon. Venus, now nearly on the opposite side of the sun from Earth and about as small as it ever gets, trembled like a flame in the wind. What will you see Friday morning?

Venus remains visible for another two weeks before it’s lost in the solar glare. We won’t see the planet at all for more than a month until it returns to the evening sky around Thanksgiving in November. Watch for it to shake and shimmy its way up the western sky until fattening up around Christmas.

3 Responses

    1. Profile photo of astrobob

      Watching the video, that’s also what I thought at first. I suppose it’s possible a plane reached an air layer where the contrail wouldn’t be visible but a meteor seems more likely. What a catch!

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