A rendezvous with Venus and Jupiter

Saturn, Venus, Mercury and the Moon – October 9, 2009, the lunar crescent joining Saturn, Venus and Mercury at dawn. Photo: Stefan Seip/TWAN; reproduced with permission.
Credit: Stefan Seip/TWAN; reproduced with permission.
Saturn, Venus, Mercury and the Moon
A rendezvous with Venus and Jupiter

In the next few days, the two brightest planets will get together at dawn for one of the most impressive encounters you can observe. What’s happening is, Venus and Jupiter will gradually get closer to each other until August 18, at which point they’ll be separated by no more than a quarter of a degree in the sky.

Celestial rendezvous

Close encounters between celestial objects (“conjunctions,” in astronomers’ jargon) are not rare events: even our distant ancestors observed the cyclic trajectories of bright stars across the night sky. At regular intervals, sometimes decades apart, they saw them converge in the sky, then drift farther apart from one another.

After thousands of years of observation, we now know that the repetition of conjunctions depends on the objects involved and on how close the stars are in the sky. For example, conjunctions between Venus and Jupiter happen an average of every 13 months, while conjunctions between Jupiter and Saturn take place only every 20 years. But whatever their frequency, planetary conjunctions are phenomena to monitor, because they allow us to track the movement of the planets in the solar system.

The Venus-Jupiter conjunction: how to watch it

To follow the progression of this conjunction, the most beautiful of the year, first locate Venus and Jupiter in the days leading up to mid-August. The two planets are easy to see at dawn, around five in the morning, just above the east-northeast horizon. If the horizon is clear of clouds, they’ll appear as two shining stars: Venus, the brighter of the two, will be sitting above and to the right of Jupiter. Closer to the Sun, Venus moves through space at 35 kilometers a second and completes its orbit in 225 days. Jupiter, much farther away, takes close to 12 years to complete its immense orbit at a speed of 13 kilometers a second.

This difference in velocity will become apparent over subsequent mornings, as Venus plunges quickly towards the Sun and gets closer to Jupiter, which will slowly climb to meet it. The gap between the two planets will shrink by about one degree per day, until August 18: on that morning they’ll be only a quarter of a degree from each other – barely half the diameter of the full moon! The day after, the two planets will move apart once more, as Jupiter climbs higher and higher in the sky and Venus sinks down to the horizon.

And now, the threesome

On the morning of August 23 the waning Moon will join the two bright planets, adding to the beauty of the scene. Since the Moon will be very close to the end of its 29-and-a-half-day cycle, the crescent will appear as a slender arc of light suspended in the dawn sky, six degrees to the right of Jupiter and Venus. The fine crescent can be admired with binoculars, in addition to revealing a delicate earthshine – the light of the Sun reflected by the Earth bathing the otherwise dark portion of the lunar disc. This combination of the Moon and the planets suspended in the colored sky of dawn is one of the most wonderful celestial spectacles there is!

For more information on observing planets, constellations, and other astronomical phenomena
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