Conjunctions and Oppositions

One of the great lessons of astronomy is that appearances can be deceiving—things are not always what they seem to be. What appear to be pin pricks of light too dim to see in the daytime are in fact enormous nuclear furnaces hundreds of time more luminous than our sun. The sun is a rather ordinary star made special solely by virtue of its proximity.

This is the case with those brighter denizens of the sky as well—the moon and the planets. We can say that they outshine the brightest stars, but even that isn’t quite accurate. They do not shine with their own light, but just reflect the sunshine that hits them. And they look bright only because they are thousands or millions of times closer to us than the stars.

When they appear close to each other in our sky, as they do now, the third dimension of distance is not at all obvious. Look at this picture taken in Manila on the evening of February 26, 2012:

Jupiter is at the top, the crescent moon in the middle, and Venus at the bottom. At the time this image was taken, however, the actual distances from Earth were quite different! The moon was a neighborly quarter of a million miles away, Venus a more distant 100 million miles or so, and Jupiter almost 500 million miles away. They appear to be near each other only from our point of view.

So let’s take a different point of view. Let’s go far above the solar system and look down on it from above the Earth’s North Pole.

This shows the orbits of the five innermost planets. The Earth is the third of these, and you can see that if we look in the direction of Venus (the second planet from the sun), Jupiter (the fifth planet) is pretty much in the same direction. All the planets orbit the sun in a counter-clockwise direction as viewed from this perspective, and that means that Jupiter and Venus are drawing closer together—as viewed from the Earth. When two planets come close together in the sky, it is called a conjunction. Jupiter and Venus will come within about three degrees of each other (the width of three fingers held at arm’s length) on March 13th. Between now and then they will draw ever closer in the sky. Venus is the one that will (appear to) move the most, rising to meet and then pass Jupiter in altitude above the horizon. Jupiter disappears in the sun’s glare in early May, while Venus remains as the bright “evening star” until the end of that month when it also disappears into the sun.

Look back at that diagram. The red planet Mars (fourth from the sun) should be visible if we look away from the Venus-Jupiter direction. In fact, while Venus and Jupiter dominate our early evening western sky, an ever-brighter ruddy orange planet is rising higher in the east every evening. Mars is approaching opposition, the time when it appears opposite the sun in our sky. A quick look at the diagram will reveal that this is when Mars and Earth are closest to each other, and when Mars appears in the sky all night long. If it is opposite the sun, then if the sun is highest in the sky at noon, Mars is highest in the sky at midnight. It rises at sunset and sets at sunrise. A Mars opposition recurs roughly every 26 months, and it affords the best opportunity for “close-up” telescopic views from the Earth. Mars’ opposition occurs on March 3rd, but it will be big and bright for several weeks thereafter.

Enjoy the show!

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Posted in Planets, Solar System
2 comments on “Conjunctions and Oppositions
  1. Anne Nash says:

    It’s going to be a really sad day for the rest of us when you retire. I hope you keep doing star struck for awhile.

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