North Hawaii News Articles from CFHT

On Stage this March -- The Planetary Ballet

This month, you have the opportunity to watch a particularly nice example of the slow ballet of the planets. The best time to watch the show is at the beginning of the night, just about 7pm, when the sky is getting dark and the planets in question have not yet set. The best place to watch is the west side of the island, where the setting planets can be seen the longest. Viewers on the east side of the Big Island may find the events near the end of the month blocked by the bulk of Mauna Kea.

At the beginning of the month, the show starts with Saturn, Jupiter and Mars clearly visible in a line in the western sky after sunset. As you look West, while the sky fades to a deep blue, you will see these three players pop on stage one by one. First Jupiter, the brightest, will become visible about 45 degrees above the horizon. A little later, Saturn, followed by Mars, will make their appearances. Saturn will be higher in the sky than Jupiter; their separation will be about the same as the width of your hand at arm's length. Mars will be lower in the sky. At 7pm, it will be about half-way between Jupiter and the horizon - assuming the horizon is not blocked by Mauna Kea! Most people are aware that Mars is reddish, but if you look carefully, you may notice that Saturn and Jupiter have colors of their own as well. Saturn has a yellowish color, while Jupiter is also slightly reddish, though not nearly as strikingly red as Mars!

Stars have colors as well. When the sky is fully dark, by about 8pm or 9pm, two of the brightest and reddest stars will be easy to spot directly overhead. These two are known by their Arabic names, Betelgeuse and Aldebaran, and they look very similar. Both are red because they are quite cool, but these two stars are actually very different in several important ways.

First, Betelgeuse is 10 times further away, which means it is intrinsically 100 times brighter than Aldebaran, if viewed from the same distance. The difference in brightness comes from a difference in size: Betelgeuse is roughly 40 times the diameter of Aldebaran. Both are large stars compared to the sun. If we replaced the sun with Aldebaran, it would be about the size of a basketball held at arm's length. Betelgeuse, however, would not only engulf the Earth, it would fill all of the space out to Jupiter as well! Another difference between these stars is in their masses. Betelgeuse is a rather massive star, several times heavier than our sun, while Aldebaran is not so heavy, being only about the mass of the sun, or somewhat less.

Both stars are nearing the end of their lives, but the difference in their masses means they will end their lives very differently. Aldebaran will die in a somewhat gentle fashion, puffing out a big cloud of gas and leaving a core that will slowly cool down and vanish. Betelgeuse, on the other hand, will explode in one of the most spectacular astronomical events, a supernova. For a couple of weeks, Betelgeuse will outshine the moon, then fade away. These exiting events will probably not take place for several thousand years. In the meantime, we can be entertained by the somewhat more subtle display of the planets.

As the month goes by, Mars will slowly approach Jupiter and Saturn. At the same time, Jupiter and Saturn will be moving closer together as well. On the 9th and 10th, there will be particularly nice views as the young crescent moon passes by the group. On the 9th, the moon will be close to Mars and Jupiter, and on the 10th, it will be almost between Jupiter and Saturn. By the end of the month, Jupiter, Saturn and Mars will form a short line only 10 degrees long -- that is the width of your hand at arms length.

The ballet continues into April, as the three planets get even closer together. Mars catches up to Jupiter on April 7 and passes Saturn on April 15th. By that time, the group of planets will be getting hard to see as they start to set only about and hour and a half after sunset. They may be blocked by clouds or heavy Vog before the sky is dark enough for them to be visible. Nonetheless, at least until the first week of April, the planetary ballet should be easy to watch. The best way to notice the subtle motions of the planets is to look at them most nights over the course of the next month or so. Compare their positions relative to each other, to the moon, and to the fixed stars, and you will start to see the motions of the planets.

It may not be quite as action packed as prime-time TV, but the planetary show is free of advertising and on every night in your neighborhood. For thousands of years, this show was the only show in town!

Eugene Magnier