This is it. What we’ve all been waiting for. Mars and Earth will pair up on the same side of the sun Sunday (May 22), putting the two planets closer to one another than anytime in the past 10 years. The special event is called an opposition because Mars lies directly opposite the sun in the sky. As the sun sets in the west, the Red Planet rises in the east and remains visible the entire night. Earth’s right in the middle, directly between the sun and Mars.
Oppositions happen about once every two years. At each, Mars lies at a different distance from Earth due to that planet’s much more eccentric (more strongly elliptical) orbit. Some oppositions are rather distant, with Mars not nearly as bright or large as it is during a closer one. Sunday night is one of the better opposition even if the Earth to Mars distance of 47 million miles sounds large to us. When you consider that the planet’s distance from Earth ranges from 33.9 millions miles (54.6 million kilometers) to 249 million miles (401 million kilometers), 47 million sounds almost cozy.
Although opposition for most outer planets coincides with the date of closest approach, that’s not true in the case of Mars. If Mars is moving away from the Sun in its orbit when Earth laps it, closest approach occurs a few days before opposition. But if the planet is moving toward the Sun when our planet passes by, closest approach occurs a few days after opposition. This time around, Mars is headed sunward, so the date of closest approach of the two planets occurs on May 30.
It’s all goes back to Mars’ more eccentric orbit, which causes even a few days worth of its orbital travels to make a difference in the distance between the two planets when Earth is nearby. On May 22, Mars will be 47.4 million miles away vs. 46.77 million on the 30th, a difference of about 700,000 miles.
Either way it’s close, and close means brighter and larger than normal. Mars will reach magnitude –2.0 late this month and swell to 18.6 arc seconds across. That’s much too small to see as a disk with a naked eye but plenty big enough in even a small telescope magnifying around 60x or higher. My students and I spotted it low in the southeastern sky last night around 10 o’clock. Despite the haze from forest fires, Mars caught everyone’s attention.
Not only is the planet bright, it’s in fun company in the constellation Scorpius the scorpion. Three fingers to its lower left, the similarly-colored star, Antares, glares from the scorpion’s heart. Four fingers to the left of the planet you’ll see another bright “star”, the planet Saturn. Get this. The Full Flower Moon slides into the scene Saturday night and together with the two planets and Antares forms a striking diamond of celestial objects. Don’t miss it!
If you’ve been watching Mars closely over the past few weeks, you may have noticed it’s moving in the wrong direction — west instead of east. As seen from the northern hemisphere, planets beyond the Earth orbit around the sun from west to east across the sky. Most of the time. For a few months centered on opposition however, outer planets such as Mars appear to slow down, briefly stop and then “drive” in the opposite direction or west.
This weird behavior is called retrograde motion, and it’s quite easy to see with Mars, which will be retrograding through early July before turning around and heading back east. Keep an eye on it, referencing the planet’s position with any of a number of stars in the Scorpius area. If you follow closely, you can see it move night to night.
Of course planets don’t stop and turn around. It’s all sleight of hand thanks to the planet you’re standing on. Shortly before opposition, as the faster Earth passes slower Mars, Mars appears to slow down, stop briefly and then “drive in reverse.” You can see a demonstration of retrograde motion when passing a car on the freeway. As you pull into the left lane, steal a glance through the passenger-side window. When you do, you can’t help but notice the car you’re passing appears to slide slowly backwards from your perspective.
Unlike a freeway, the planets travel in near-circular orbits, so some months after opposition, as Earth turns away from the more distant planet, Mars will “catch up” and begin moving east again from our perspective.
Mars will be visible for many months to come but will gradually fade after May 30th as the two planets part ways. Find it now and follow its travels across the zodiac. I guarantee a long, satisfying ride that will reveal how the outer planets – and Mars in particular – move across the sky. If you’re not familiar with the dozen zodiac constellations, you will be after Mars finally fades into the sunset next May.
Tomorrow, we’ll take a look at what Mars offers for telescope users.