The dance of the planets


The coming weeks will see an extraordinary set of close encounters of the three brightest objects in the nighttime sky: the planets Luna, Venus and Jupiter.

From the point of view of our ancient forbears, the moon was indeed considered a planet. In fact, what we think of as planets has changed remarkably over the millennia of human civilized history. To the ancient Greeks, the stars were fixed with respect to each other into constellations. The planets were “planites” (plan-EE-teez), wanderers through the constellations.

The planets were apparently able to move at will like, well, gods. Our earliest conception of the planets, sun and moon included, was that they were deities. It’s no coincidence that the planet Jupiter is also the name of the king of the gods or that Venus was the goddess of love and beauty. Astrology, the belief that the motions of the planets somehow influences or predicts human behavior, is founded on the belief in ancient gods.

Ancient astrologers and astronomers alike had fits trying to predict the exact positions of the planets against the starry background. They assumed that the planets rotated around Earth in perfect circles and at uniform velocity. Such assumptions got the planets pretty close to their actual positions in the sky, but in astronomical terms, close isn’t close enough.

Johannes Kepler came along and revolutionized our ability to predict planetary positions by accepting the Copernican model. Earth and the other planets orbited the sun. However, he had two other changes in mind. The orbits were not circular. They were ellipses, stretched-out circles. Also, their motions were not uniform. They speeded up a bit as they got closer the sun and slowed down as they withdrew.

Thanks to Kepler, I can predict with utter accuracy that Venus will appear to approach Jupiter in the coming weeks and pass within a hair’s breadth of it on the evening of June 30. Along the way, a pretty crescent moon will enter the tableau on June 19 and 20.

The best time to start observing is tonight if it’s clear. You’ll need a western horizon relatively clear of trees and buildings. Go outside just after sunset and wait for Venus to appear in the growing twilight as the sky turns from light to darker blue. She will appear pretty high in the west as a brilliant point of purest white light. Binoculars will reveal a tiny disk only about half illuminated. Yes, very much like the moon, Venus has phases.

As darkness grows, look for fainter Jupiter up and to the left of Venus. Binoculars will show three or four of its bright “Galilean” moons, so named because Galileo was the first too see them in his small telescope just over 400 years ago.

Every clear evening you get, go out and watch the fun as Venus approaches Jupiter. As Venus moves, it will also pass in front of the beautiful Beehive star cluster, a spread-out collection of stars in the constellation Cancer, the Crab. Wait as long as you can to point your binoculars. Maximum darkness helps to increase the Beehive’s visibility. Venus will make its closest approach to the Beehive during the evenings of June 13 and 14.

In the meantime, Venus and Jupiter are getting closer and closer. By June 27, they are about four moon diameters away from each other and will stay that way for a week or so. (Remember, Venus is the brighter one.) On the evening of June 30, Venus will pass within a lunar diameter of Jupiter before their distance begins to widen again.

The June 30 close approach should make for a spectacular binocular view. Jupiter’s moons will still, of course, be present. Venus will have shrunk to about a 34-percent illuminated crescent. Seeing them both in the same binocular field should be a sight worth remembering.

During July, Jupiter will drop out of the scene. However, keep observing Venus in binoculars. It will be getting closer to Earth and therefore will appear larger each day. At the same time, its crescent shape will shrink to razor thin by the beginning of August.

Such close conjunctions of Venus and Jupiter seem to come along every couple of years, but they rarely come as close as they will in late June. Some astronomers, notably the devout Christian Johannes Kepler, attribute the Star of Bethlehem story to such a close approach of Venus and Jupiter in 3 BCE. To ancient Greeks and Christians alike, such events are spectacular enough to elicit a sense of religious awe.

Will this latest approach be so memorable? Only time and cloud cover – or lack thereof — will tell the tale.