From the USM Southworth Planetarium
“Just right”
THE DAILY ASTRONOMER
November 3, 2009
Minding Mars
This is not a hoax.
For the next few months we’ll watch as Mars becomes brighter in the sky as the planet moves closer to Earth.
We repeat: this is not a hoax.
One can find Mars for the rest of the year in the constellation Cancer the Crab. Being presently as bright as a first magnitude star, Mars is easy to find amidst the crab’s faint component stars. Both Cancer and Mars are visible in the late evening eastern sky tonight.
During the next few months, observers can watch Mars moving eastward until it rises around the time of sunset toward the end of January, when it reaches opposition. Around this time, Mars will also reach its closest approach point to Earth during this orbit.
Let’s engage in some explanations.
When a celestial body is on the other side of Earth relative to the Sun it is said to be in “opposition.” As one can determine by examining the graphic, when a planet is at or close to opposition, it will be at its closest point to our planet. As a consequence of this proximity, the planet will also appear at its brightest since a planet’s magnitude depends partially on its distance from the Sun and its distance from Earth.
So, as Mars draws closer to our home planet, we’ll see it gradually brighten until it appears as a bright, crimson-hued star in the eastern evening sky.
Mars will be at opposition on January 29, 2010, but reaches its closest approach to Earth on January 27th. That seems curious, doesn’t it? Looking at today’s graphic may make one wonder why a planet isn’t at its closest point to Earth precisely at opposition.
The reason for this apparent discrepancy pertains to the motions of both planets. Earth will reach perihelion -its closest point to the Sun- on January 3, 2010. After this event, Earth’s distance from the Sun will continually increase until our planet reaches its greatest distance from the Sun (aphelion) on July 6th.
Mars, meanwhile, is already moving away from the Sun. It has been ever since it reached its own perihelion on April 21st. So, as Earth moves toward the line between Mars and the Sun, Mars will be moving away from the Sun and Earth. A couple of days before the actual opposition, Earth and Mars will be as close to each other as possible during this orbit. That distance, incidentally, is about 61 million miles. Compare this approach to the now legendary closest approach in August 2003, when Earth and Mars were only 34.5 million miles apart.
Mars was so close to us during that August 2003 opposition because it was also close to its own perihelion (August 30, 2003). In January 2010, Mars will be close to its aphelion, which it reaches on March 30, 2010. Therefore, even though the planets are close, they are still millions of miles farther away from each other than they sometimes can be when Mars is at perihelion the same time it is at or around opposition.
