Observer's Notebook for April, 2000

Planetary Alignments to Remember... and Anticipate

by John Rummel

My article last month on planetary alignments got me to thinking about great planetary displays of the past and future. I well remember the incredibly close grouping of Venus and Jupiter last February, and the nice morning trio of the crescent moon, Venus and Jupiter the April before that. As you might guess, planets have casual meetings in our skies all the time, but for these past few years, we're talking about two or three heavenly bodies meeting for an evening or two. What about groupings involving more than just two objects?

Just a few years back, June 15 of 1991, Jupiter, Venus and Mars, along with the crescent moon, were very tightly grouped on the western horizon just after sunset. How rare is such an event? A quick search with Voyager II software found 107 such occurrences between the year 1 and 2000. That's an average of about every 18 1/2 years with most of the events being far enough from the sun that they're easily observable. Other groupings of three naked-eye planets and the moon show similar frequencies with the exception that if Mercury is included in the group, the percentage of groupings being easily observable goes down. Mercury never strays more than about 28 degrees from the sun, so groupings including Mercury are much more likely to be too close to the sun for easy visibility.

What about even larger massings of naked eye objects? If we take all five naked eye planets, Mercury, Venus, Mars, Jupiter, and Saturn, and throw in the moon, we have six of the seven classical solar system objects. We'll leave the sun out because we're looking for planetary groupings far enough from the sun that they're observable. Like this May, when the sun is included, it becomes pretty much a non-event for sky-watchers.

Voyager II found 51 such events in the 4000 year period between 2000 BC and AD 2000, an average of one occurrence every 78.4 years. However, the vast majority of these are unobservable due to the sun's intrusion. In fact, of these 51 events, only 14 were even marginally observable, bringing the average down to once every 285 years, so chances are good that none of us reading this will ever get to observe such an event.

Just out of curiosity, what is the best planetary grouping ever? My reading and research have turned up two remarkable events. On the morning of February 24th, 1953 BC, all five naked eye planets were within 4 degrees of each other just before dawn. By March 2nd, the grouping wasn't as tight, but the crescent moon joined the show. History doesn't contain any record of anyone having observed this remarkable grouping, but it must have been breathtaking for anyone who was paying attention.

Above: Looking southeast just before dawn on March 2, 1953 BC. From left to right, Venus, Mercury, Mars, Saturn, and Jupiter, with the crescent moon.

Looking west, just after sunset on September 8, AD 2040. From left to right, Mars, Venus, Saturn, Mercury, and Jupiter with the crescent moon.


The other "best ever" planetary grouping hasn't happened yet, but odds are at least better that you'll live to see it. On the evening of September 8th, 2040, all five naked eye planets and the crescent moon will be in a grouping just 9 degrees wide low in the western sky. While not quite as tight as the 1953 BC event, it will be quite memorable. I've got it marked on my calendar. How about you?


Back to Astronomy Main Page