If you've been out in the early evening lately, perhaps you've noticed Jupiter, which has been dominating the evening sky the past several months, now sinking closer to the setting Sun in the west. And if you turned around and looked behind you, perhaps you've also noticed another star rising after dark and dominating the sky in the east, that "star" being the planet Mars.
Traveling nearly 67,000 miles per hour, Earth orbits the Sun once each year. Mars, the next planet out from the Sun, moves only 54,000 miles per hour, has further to travel, and thus takes nearly two Earth-years to orbit the Sun.
Since we speed around the Sun more quickly, we regularly pass between Mars and the Sun about every two years. When we do, Mars is on the opposite side of Earth from the Sun, which astronomers call opposition -- and this is about to occur Jan. 29.
At opposition, Earth and Mars pass nearest each other, making Mars appear larger and brighter than usual. So now and for the next few weeks, Mars outshines all the brightest stars (except Sirius which is now in the southeast in the early evening.)
If Earth and Mars orbited the Sun in perfect circles, Mars would appear the same size and brightness at each opposition. But since their orbits are elliptical, at some oppositions Earth and Mars pass nearer than at others. On average we pass within 48 million miles (rounding to the nearest million), but the distance can be as little as 34 million miles or as much as 64 million miles.
This time around, we're passing at 62 million miles, so this is not one of Mars' more spectacular oppositions although it will still be well worth noting. (Perhaps you recall the excitement in August 2003 when Mars passed less than 35 million miles and was extraordinarily bright -- that was pretty spectacular.)
All the planets further out from the Sun come to opposition regularly. The period between Jupiter's oppositions is about 13 months, and for the more distant Saturn, Uranus, and Neptune, just over a year.
At opposition, planets rise around sunset, are up all night, and set around sunrise. And since they are then at their largest and brightest, the few weeks before and after opposition are the best times for observing them.
By coincidence, on the night of Mars' upcoming opposition, it has a companion to escort it across the sky -- the almost full Moon. And then the first week of February, the Red Planet passes near the lovely Beehive star cluster. They will be in the same binocular field of view several nights in a row -- a sight you won't want to miss.
• Sky Calendar.
* Jan. 30 Sat.: The full Moon is called Old Moon and Moon After Yule.
* Feb. 2 Tue.: Commonly known as Groundhog Day, today is also Candlemas, a cross-quarter day celebrating the middle of winter.
* 4 Thu. morning: The gibbous Moon is below Virgo's brightest star Spica high in the south.
* 5 Fri.: The Moon is at 3rd quarter.
* 7 Sun. morning: The crescent Moon is to the upper right of Scorpius' brightest star Antares low in the southeast.
* 11 Thu. morning: The crescent Moon is to the upper right of Mercury very low in the east southeast at dawn, and to the planet's lower left the next morning.
• Naked-eye Planets. (The Sun, Moon, and planets rise in the east and set in the west due to Earth's west-to-east rotation on its axis.) As evening twilight ends, Jupiter is setting in the west as Mars is rising in the east. At the first light of dawn, Mercury is very low in the southeast, Saturn is in the southwest, and Mars is in the west.
Stargazer appears every other week. Paul Derrick is an amateur astronomer who lives in Waco. Contact him at 918 N. 30th, Waco, 76707, (254) 753-6920 or firstname.lastname@example.org. See the Stargazer Web site at stargazerpaul.com.