A Planetary Full House
Observer's Notebook for April, 2002

by John Rummel

There are five planets that can be seen without a telescope. These so-called classical planets are the ones that have been known since the beginning of recorded time. The end of April and beginning of May will present an opportunity many sky watchers have never seen: all five will be visible in the western sky after sunset.

Smaller groupings of the visible planets happen virtually every year. An appearance of all five naked-eye planets is rarer, but not as rare as you might think.

The question is eventually raised, "when was the last time this happened." Such questions are difficult to answer because, like snowflakes, no two planetary groupings are exactly the same. To help answer the question though, I took this April's grouping of all 5 naked eye planets in the western sky after sunset, and searched for similar occurrences. The conditions of my search were that 1) all five planets had to be in the same part of the sky (I limited my search to the same as April 20th grouping, about 50 degrees – about twice the apparent length of the Big Dipper) and 2) The planets had to all be far enough away from the sun that you could actually see them after sunset or before sunrise.

This last factor is the key when the goal is observability. Many, many planetary groupings occur when the sun is right in the middle of the pack. The result is that none (or very few) of the planets are visible because they are lost in the glare of the daytime sun, sunrise, or sunset.

It turns out that opportunities for mass groupings occur at regular intervals about every 20 years. This schedule is set by the two slowest moving of the five planets: Jupiter and Saturn. These two giant planets only meet together in our sky at regular intervals separated by about 20 years. Thus, a visible grouping of all 5 naked-eye planets is limited to the few years surrounding a Jupiter-Saturn meeting. For the last few centuries, these meetings of the giant planets have happened during every other zero-year decade, 1920, 1940, 1960, and so on. The last such Jupiter/Saturn meeting was in May 2000, during the so-called "Grand Conjunction." This was indeed a wonderfully close grouping of all five naked eye planets, but it was a complete bust for observers. The sun was right in the middle of the grouping, and nobody saw a thing. This month's grouping of the five classical planets is part of that 2000 Jupiter-Saturn meeting. Though the giant planets are slowly drawing apart, they are still close enough to allow for a 50 degree grouping of the five.

Observing Tips

It's important to remember that a grouping of planets like the one this month doesn't happen all in one night, like an eclipse or meteor shower. It unfolds gradually over a period of weeks.

For the backyard observer, here are tips for watching this event:

- By the beginning of April, the lineup is almost complete. Just after sunset, Jupiter is highest in the western sky, and brilliant Venus is lowest. Between the two are the dimmer Mars and Saturn. If you can spot the stars of Orion's belt after sunset, look just to the right (about 3 fist widths) for Saturn, Mars, and the star bright Aldebaran.

- On about April 22 or 23, Mercury begins to emerge from the glare of the sunset. As Mercury gets a little higher each day, it will become visible just below the beacon of Venus (see first diagram)

- By May 4th, Venus, Mars and Saturn will be in a tightly grouped triangle. Venus will be visible long before the others due to its brightness. The trio will set about 90 minutes after the sun. May 4th–6th will be the best days to view this sub-grouping.

- After May 5th, Mercury quickly drifts back into the glare of sunset. Your best window of opportunity for viewing all five planets is between April 25 and May 10.

- On May 14th, the crescent moon will be very close to Venus in what will make a very pretty grouping. Again, the best time to look is about an hour after sunset.

(Images created with Starry Night Pro astronomy software)

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