Mars rises as Jupiter sets

Published 2:59 pm Wednesday, January 13, 2010

On clear dark nights away from city lights, you will often be greeted by a panorama of sparkling stars. If one is exceptionally bright, chances are it may be an imposter a planet masquerading as a star. But its unlikely the impersonator will be Mars.

But, you ask, isnt Mars our next-door neighbor? Shouldnt it be a frequent visitor to the night sky and easy to find? How much harder is one planet to find than another? Which are hard and which are easy? How often can we see them? To answer these questions, lets take a quick look at all eight planets (earth plus 8 = 9) and compare them in terms of what we might call their visibility index.

The visibility of a planet depends primarily on four factors: 1) the planets actual size, 2) its distance from earth, 3) its reflectivity, and 4) its orientation (is it sometimes obscured by the sun?)

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The outer-most planets, Uranus, Neptune and Pluto, are located so far from the sun that by the time their light reaches us, it has mostly evaporated.

Uranus was not identified as a planet until 1781, Neptune in 1846 and Pluto in 1930. None of these three can be an imposter.

The next planet in toward the sun is Saturn. Saturn is a giant planet. Its massive system of rings spans more than 20 earth diameters and is highly reflective. This combination of size and reflectivity make Saturn a conspicuous visitor to the night sky even though its average distance from us is more than one billion miles. Saturn is therefore, quite often mistaken for a star.

Jupiter is next, half the distance of Saturn. A light source half the distance of another is four times brighter, so all things being equal, Jupiter should be four times brighter than Saturn.

But nothing about Jupiter is equal. If it could be put on scales, Jupiter would weigh twice as much as all the other planets combined. On the average, Jupiter is six times brighter than Saturn, so it has a hard time pretending to be a star.

Planets farther from the sun than earth are called superior planets.

Superior planets remain visible throughout the year except for the brief period when they transit behind the sun at which time the planet is said to be in conjunction with the sun.

When a planet is aligned so that it is opposite the sun in our sky, the configuration is called opposition. Superior planets are best observed at opposition as they are then closer to earth than at any other time.

After Jupiter comes Mars. Mars will reach opposition in two weeks and will remain a splendid attraction for several months. Its uniquely red-orange color is unmistakable against the backdrop of winter stars. But by mid-April, Mars will be on a fast track to another part of the solar system and quickly fade from view.

Earth is next in line, and inward from earth is Venus. Planets inside earths orbit are called inferior planets and both Venus and Mercury fit that description.

Because inferior planets are very close to the sun, they are visible only for short time periods, usually one or two hours before sunrise or after sunset. They are the morning and evening stars and are never seen late at night.

Venus and Mercury make up for their abbreviated stage time by having very fast orbital speeds which causes them to pop up on either side of the sun on a frequent basis.

They move so quickly their time behind the sun lasts only a few days.

If the average star is a flashlight, Venus is a lighthouse. At its brightest, Venus is 16 times brighter than Sirius, the brightest star in the sky, and six times brighter than any other planet. Venus is also the most reflective of the planets.

Venus is now opposite the sun so it cant be seen. However, it will soon reappear in evening twilight as the evening star, and will continue to brighten in the western sky until early next fall when it passes in front of the sun and re-appears shortly thereafter as the morning star.

Mercury is harder to find than Venus because it is closer to the sun and the window of opportunity to see it closes quickly. Though Mercury occasionally rivals Jupiter in brightness, the planet remains elusive because it usually sets before twilight completely ends. According to legend, even the great Copernicus never saw Mercury.

Now lets return to Mars.

The orbital period of Mars (its year) is 687 days compared to 365 days for the earth. After 365 days, earth has returned its starting point while Mars has traveled just over half way round and is hidden by the sun. Meanwhile, its distance from earth has increased by a factor of five, so its light has diminished by a factor of 25. It takes another 365 days for the earth to begin catching up with Mars again.

Therefore, Mars is a bright and easy target less than half the time, the few months before and after opposition. At all other times, it is faint and unremarkable.

Mars now rises at sunset, sets at sunrise and remains clearly visible all night. As Mars rises in the east, Jupiter will set in the west. You can clearly trace the plane of the solar system by viewing these two planets at once in opposite parts of the sky.

Join the astronomers at FENCE tomorrow, Saturday, January 16, after sunset, where Mars will be the main attraction. And dont forget to bring your binoculars!~ Starry Messenger written by Jim Cooper