THE MARTIAN CYCLE
Depending on when you look at it, Mars's apparent size can vary greatly—from 4 to 25 arc-seconds. A typical apparition begins with Mars low in the east in the glow of morning twilight. As weeks pass, the planet grows. It moves eastward against the stars and does not rise out of the dawn quite as quickly as they do. Earth, , orbiting faster than Mars, begins to overtake it, and Mars's apparent eastward motion slows and stops.
Mars then spends about 11 weeks moving westward against the background stars before halting once more and resuming eastward motion. In the middle of the westward (or retrograde) part, Mars reaches opposition and best visibility around niidnight, and is at its maximum apparent size for the apparition.
Finally, over the following months, Mars slips into the early evening sky and eventually disappears in the twilight glow. After remaining hidden in the Sun's glare for several weeks while it passes conjunction, Mars reemerges at dawn to begin its next apparition.
Mars comes to opposition about every 780 days, or 2 years 7 weeks. Because it has an eccentric orbit, Mars's distance from the Earth and the Sun at opposition varies, so its apparent size and brightness also change. The most favorable opposition is when Mars lise closest to both the earth and the Sun. Such oppositions are called perihelic, and they occur when opposition is in July, August, and early September— about once every 17 years. At these times, Mars is just 35 milion miles (56 million km) from Earth, with an apparent size of about 25 arcseconds and a magnitude of—2.6. At aphelic oppositions (in January, February, and early March), Mars is 63 million miles (101 million km) from Earth. Its disk is only about 14 arcseconds wideg and it shines at magnitude -1.0 more than four times dimmer.