Introduction to Mars
The Red Planet
Mars is also known as the Red Planet , although its true color varies from rusty orange to gray to white. Some casual Earthbound observers mistake it for a red-giant star. However, because of its significant apparent diameter, it does not twinkle as does a star. In ancient mythology, Mars was the god of war. The planet has two moons, named Phobos (Greek for “fear”) and Deimos (Greek for “terror” or “panic”).
Conjunctions And Oppositions
Mars never passes between Earth and the Sun because the orbit of Mars lies entirely outside that of Earth. Mars occasionally lines up with the Sun when it is exactly opposite the Sun from us. This is called conjunction . There is no need to use the word superior because there is only one kind of Martian conjunction. (With Venus and Mercury, there are two kinds, inferior and superior, as you know from the Venus and Mercury study guides.)
Mars does not pass through significant phases as do Mercury, Venus, and the Moon. We always see Mars with most of its face lit up by the Sun. However, the brightness of Mars in the sky does vary greatly. It is dimmest when it is at and near conjunction. When it is very close to conjunction, Mars is invisible because its wan glow is washed out by sunlight. Earth travels more rapidly around the Sun than does Mars, so this unfavorable condition never lasts for long. After a conjunction, Mars begins to show itself in the eastern sky before dawn. As time passes and Earth begins to catch up with Mars, we get closer and closer to the Red Planet. As this happens, Mars appears earlier and earlier in the predawn hours, and its brilliance increases. Eventually, Mars reaches a position opposite the Sun so that it rises when the Sun sets, is visible all night long, and sets at sunrise. Then the Red Planet is at opposition , and it rivals Jupiter in brightness. As seen with the unaided eye, Mars at opposition is a more attention-getting sight than any other planet except Venus.
Opposition is the best time to view Mars, but some oppositions are better than others. This is so because the orbit of Mars is far from a perfect circle around the Sun. While Earth’s orbit only varies a percentage point or so either way from perfect circularity, Mars follows a decidedly elliptical path with the Sun at one focus. The best oppositions, in terms of viewing Mars from our planet, occur when three things happen at the same time:
- Mars is at opposition.
- Mars is at perihelion (closest to the Sun).
- Earth is at aphelion (farthest from the Sun).
This can only take place during the northern hemispheric Earth summer, especially during the month of July, because that is when Earth is at aphelion (Fig. 6-1). The ideal state of affairs happens only about once every 15, 16, or 17 years.