Diana Hannikainen, Observing Editor, Sky & Telescope
617-864-7360 x22100, diana@SkyandTelescope.com
J. Kelly Beatty, Senior Editor, Sky & Telescope
|Note to editors/producers: Publication-quality illustrations are available; click on the thumbnails at the bottom.|
After a slow crawl across the predawn darkness earlier this year, Mars is finally moving into the evening sky — just as it comes its closest to Earth in 15 years. According to Sky & Telescope magazine, the two planets’ centers will be separated by just 35,784,871 miles or 57,590,017 km on July 31st at 3:50 a.m. EDT (7:50 Universal Time).
This is the closest Mars has come to us since August 27, 2003, when the separation was 34,646,418 miles. On that date, the Red Planet was closer to Earth than it had been since 57,617 BC. (Click here for more information regarding 2003’s historic close encounter.)
Right now Mars appears especially big and bold in the night sky. Its peak brightness, as measured with the magnitude scale used by astronomers, will be –2.8. This means Mars appears twice as bright as Jupiter, which is also prominent now in the southwestern evening sky, and it will continue to outshine Jupiter until the first week of September.
“When you first spot Mars rising in the east after sunset, you’ll be startled by how bright it looks,” notes Diana Hannikainen (pronounced huhn-ih-KY-nen), Sky & Telescope’s Observing Editor. “Its pale orange color is unmistakable.” In fact, Mars really has a reddish-orange hue, caused by rust-colored iron oxides on its surface.
There are actually two related events involving the Red Planet this week:
July 27: At 1:07 a.m. EDT (5:07 UT), Mars reaches opposition. This means that the planet appears directly opposite the Sun as seen from Earth. Mars rises at sunset, remains in the sky all night, and sets as the Sun rises. Mars takes 1.88 years (687 days) to circle the Sun, and faster-moving Earth essentially “laps” its neighbor about every 26 months. The previous opposition of Mars was May 22, 2016, and the next one will be October 13, 2020 — but on those dates the Red Planet will be considerably farther away than it is now.
This year’s opposition is special because it also occurs close to when Mars reaches the point in its orbit that’s closest to the Sun, called perihelion. The planet’s orbit is distinctly out of round (its eccentricity is 0.09, versus 0.00 for a perfect circle), so at times Mars can be up to 13 million miles (21 million km) closer or farther from the Sun than average. This year the Red Planet reaches perihelion on September 16th.
On the night of its opposition, Mars will be joined in the sky by a full Moon — which, coincidentally, undergoes a total eclipse that day. However, the eclipse occurs during daylight hours for the Western Hemisphere and thus won’t be visible from the U.S. or Canada. (Click here for more information about the eclipse.)
July 31: Because Mars still edging closer to the Sun, roughly 60,000 miles (100,000 km) per day, it continues to come slightly toward Earth for a few days after opposition. So the two planets actually come closest on July 31st. Their surfaces (not centers) will be 35,778,704 miles (57,580,243 km) apart when nearest.
In the sky, Mars is positioned in southern Capricornus, the zodiacal constellation that’s often represented as half goat and half fish. Owing to Earth’s faster orbital motion, for the next month Mars will slowly glide in a retrograde (westward) direction among the stars, toward Sagittarius.
Although Mars is especially close right now, it will remain relative low in the southern sky for viewers in the United States and Europe. “Despite its glorious girth, northern observers will pay a price during this juicy Mars apparition,” comments S&T Contributing Editor Bob King. “At most perihelic oppositions, including this one, the planet retreats to the belly of the ecliptic low in the southern sky.”
Even when it’s highest up each night, Mars is only 23° above the horizon as seen from Chicago and 13° from London. This low altitude means you’ll see Mars through a thicker atmospheric layer than if it were higher up. But those in the Southern Hemisphere fare much better: Mars will appear overhead as seen from central Chile, South Africa, and Australia.
Seen through a telescope, the planet's disk when closest will have a diameter of 24.3″ (24.3 arcseconds, with 1 arcsecond being 1⁄3600 of 1°). This is about 1% the diameter of a full Moon, so you will need a decent backyard telescope to see any features on its tiny disk. However, right now the Red Planet’s surface features are largely hidden by a global dust storm that began in late May. (Click here for more information about the dust storm.)
Don’t worry if the weather is cloudy in the coming week. Mars will remain almost as bright for several weeks, and over that span you’ll see it glide a little higher above the horizon in evening twilight. Keep an eye out as well for the bright planets Saturn, Jupiter, and Venus, which form a long arc across the sky from east to west in the hours after sunset.
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