ANN ARBOR—On March 17, the Earth will pass between Mars and the sun at a distance of 60 million miles—our closest approach to Mars until 1999. If the moon or clouds interfere with the view on the 17th, however, Mars will remain visible all month rising in the east at sunset and setting in the west at sunrise.

Located directly south of us at midnight, the planet is a bright reddish lamp hanging halfway up from the Southern horizon, according to Richard Teske, University of Michigan professor emeritus of astronomy.

“Moonlight will dim observations of Mars in mid-March,” Teske said. “But sky watchers who stay up to observe a partial eclipse of the moon beginning at 10 p.m. on March 23 should be rewarded with sightings of the red planet just above and to the right of the darkened moon.”

This month’s close encounter between Mars and Earth takes place because the two planets play a perpetual game of orbital tag, Teske explained. The more swiftly moving Earth chases slower Mars, gaining one full lap and passing once every 26 months. The next close encounter will occur in the spring of 1999.

“A view of Mars through a telescope this month reveals its glittering white north polar ice cap tilted toward the sun and toward us. It is now the summer solstice on Mars, the equivalent of our June 21st,” Teske said. “Days are longest in its northern hemisphere and shortest in the south, which leans away from the sun this month.”

Mars has seasons just as Earth does and for the same reason- Mars circles the sun, its northern hemisphere sometimes leans toward the sun, giving summer there; while at other orbital positions the north tilts away, bringing winter. Astronomers have determined that the inclination between its equatorial plane and orbital plane is 23 degrees—very nearly the same as for Earth. A day on Mars is only slightly longer than a day on Earth. The interval between one sunrise and the next is 24 hours, 39 minutes and 35 seconds.

Astronomers think the present near-equality of day length and equatorial inclination for Mars and Earth is a coincidence. Theorists predict that Mars’ equatorial tilt changes significantly over millions of years. Its current 23-degree tilt is only temporary, according to Teske. Further, Earth’s day is becoming longer as our planet’s spin gradually slows down. “Some 160 million years from now, the length of an Earth-day and a Mars- day will be equal. After that, Earth-days will continue to grow longer,” Teske said.

“An observer standing on Mars looking up at the sky would see the same stars and constellations we see from Earth. The Big and Little Dippers, Leo, Gemini and all other constellations would be visible rising in the east and setting in the west,” Teske said.

A significant difference between Mars’ sky and Earth’s sky is in the location of the rotational pole of the heavens. Earth’s rotation axis points close to a star at the end of the handle of the Little Dipper called Polaris, the North Star. With the passing night hours, the sky seems to rotate around the star. Mars’ rotation axis points at a position between Cygnus, the Swan, and Cepheus, the King.

“An observer on Mars would be aware of the sky seeming to rotate around this place,” Teske said. “This position is close to a bright star named Deneb marking the Swan’s tail, which would serve as Mars’ North Star.”

Observers on Mars would sometimes see the Earth playing the part of morning star or evening star, imitating the roles that the planet Venus enacts in Earth’s morning and evening skies. “But because Earth never gets as close to Mars as Venus gets to Earth, and because Earth reflects only half the sunlight reflected by Venus’ clouds, our planet would make a rather pale imitation. It would surely not be the grand spectacle furnished by Venus that Earthlings are privileged to witness.”