Quick Guide to the Mars Opposition of October/November 2005

by Wm. Robert Johnston
last updated 29 July 2005

For a version of this handout for the Mars opposition of August 2003, follow this link.

What is Mars?

Mars is the fourth planet from the Sun, or the next planet out from the Earth. Often visible with the unaided eye, it has long been a source of fascination.

Mars is about half the size of the Earth (in diameter). Its atmosphere is much thinner than the Earth's, and is mostly carbon dioxide. The average temperature on Mars is -45° C (-50° F), although it can range from -100° to 0° C (-150° to 30° F). Liquid water cannot exist on Mars on the surface. The time for Mars to complete one orbit around the Sun, the Martian year, is 687 days or 1.88 of our years. But Mars rotates on its axis once every 24 hours and 37 minutes, very similar to our day.

The surface of Mars is rock and dust. There are many craters from the impacts of comets and asteroids, but there are also vast plains with extinct volcanoes larger than any on Earth, a rift valley the length of the United States, and channels apparently cut by running water sometime in the past. Mars appears orange with dark brown regions of wind-blown dust. There are also white polar caps of ice and dry ice (frozen water and frozen carbon dioxide) which expand and contract with the seasons.

Five NASA robot probes have sent back pictures from the surface of Mars. They show rock-strewn plains of dust and sand dunes. The sky is pink, colored by dust in the thin atmosphere. Some pictures have shown morning frost in the winter.

Mars has two small moons, Phobos and Deimos. They are each dark irregular objects, smaller than a large city.

As late as the 1960s, many astronomers thought Mars likely to have life, given its similarity to the Earth. Robot space probes showed it to be cold and dry, however. In 1976 NASA landed the Viking probes on Mars. Their tests for living microorganisms indicated no life on Mars. But astronomers still think we might find evidence of past life, like fossilized microorganisms.

How close is Mars now and why?

Mars' average distance from the Sun is 1.5 times greater than the Earth's average distance. As the Earth and Mars each orbit the Sun, with the Earth orbiting faster, periodically the Earth catches up to Mars and is adjacent to it. This is called an opposition--when the Sun and Mars are opposite each other as seen from the Earth. Oppositions with Mars happen every 2 years and 2 months. On November 7, 2005, Mars will be at opposition.

Mars' orbit is elliptical (meaning not perfectly circular), and is more so than the Earth's orbit. As a result, Earth will actually be closest to Mars a bit earlier: at 10:20 PM CDT on October 29, 2005. Then, Mars will be "only" 69,415,600 km (43,132,800 miles) away from the Earth. This is 181 times further than the Moon, or less than half the distance to the Sun. From September to December, Mars will be close and bright.

Some of you may remember the last Mars opposition, on August 27, 2003. This was a special opposition, a perihelic opposition. About every 16 years, Earth catches up to Mars while Mars is at its closest point to the Sun. At its closest, Mars was about 20% closer to the Earth in 2003 than it will be in 2005. However, for those of us in the northern hemisphere, Mars will be higher in the sky this time. So our viewing will be just as good!

Mars as seen by the Viking 1 orbiter. Valles Marineris (Mariner Valley) cuts across the center from left to right. (Credit: NASA/USGS)

Olympus Mons, an extinct Martian volcano the size of New Mexico, as seen by the Viking 1 orbiter. (Credit: USGS)

The view from the rocky surface of Mars. This was taken by the Mars Spirit rover in 2005. (Credit: JPL/NASA)

Diagram of the orbits of the Earth and Mars, showing the positions of each planet on Nov. 7, 2005. The tick marks show each planets' positions at one month intervals. (After diagram from Heavens-Above.com)

What can I see?

In early fall Mars will look like a bright reddish-white star in the constellation Aries. In late September-early October it will enter the constellation Taurus. it will rise by 7 PM and be overhead in the southern sky around midnight (for observers in south Texas). Without binoculars or a telescope, you can't miss it: it is brighter than any star in the night sky, and doesn't twinkle.

With binoculars, you may be able to tell that Mars is a disk (not a point of light like the stars). But with a good telescope, this is a good opportunity to see surface features on Mars.

Mars would appear as an orange disk, with shaded areas of darker dust and maybe one polar cap visible. The diagram below shows some surface features that might be visible in a good telescope, depending on which side of Mars is rotated toward us. The six views cover a full rotation of Mars (24 hours and 37 minutes).

A sequence of views showing the Mars as it goes through one rotation. NOTE: if you are viewing Mars through a Schmidt-Cassegrain telescope, each view should be reversed from left to right; if you are viewing Mars through a refracting telescope, each view should be reversed from top to bottom. Also NOTE: these images were made for the Aug. 2003 opposition; the view in 2005 is slightly different.

Where can I learn more?

Your local public library (or school library for students) is a good place to read more about Mars. Two good magazines which may be available are Sky and Telescope and Astronomy.

There are many good web sites; some recommendations are:

In Brownsville, Texas, the South Texas Astronomical Society meets monthly at the Brownsville Public Library. The University of Texas at Brownsville Physics and Astronomy Department is engaged in astrophysics research and offers astronomy courses to university students. This includes its Center for Gravitational Wave Astronomy, supported by NASA and concerned with gravitational wave research, both theoretical and observational. The Brownsville Alliance for Science Education ENLACE Project in cooperation with the University of Texas at Brownsville is finalizing plans for an astronomical observatory which will be dedicated to public outreach.

© 2003, 2005 by Wm. Robert Johnston.
UTD Physics/Center for Space Science
UTB/TSC Physics and Astronomy
Last modified 29 July 2005.

UTD Physics Department -- http://www.utdallas.edu/dept/physics/
UTD William B. Hanson Center for Space Science -- http://utd500.utdallas.edu/
UTB/TSC Physics and Astronomy Department -- http://www.phys.utb.edu
Center for Gravitational Wave Astronomy -- http://cgwa.phys.utb.edu
Brownsville Alliance for Science Education -- http://www.phys.utb.edu/base/

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