For the subsequent two and a half weeks, early-morning skywatchers could have a superb alternative to identify the so-called “elusive planet” — Mercury.
Two planets are nearer to the sun than Earth is — Venus and Mercury — and are often known as the “inferior” planets. Venus orbits the sun as soon as each seven and a half months and beneficial properties a complete lap on the slower-moving Earth each 584 days. Venus is a night object for about 9 months till it passes behind the sun, then a morning object for 9 months because it strikes between the sun and Earth. And proper now, Venus is a really distinguished, albeit low, beacon within the southwest sky quickly after sundown.
Related: 10 strange facts about Mercury
Now is Mercury’s time
But whereas many individuals have seen Venus, not so many have ever seen Mercury. Scarcely greater than half the space from the sun as Venus is, Mercury would not enterprise removed from the sun within the sky, dodging out into view within the night twilight low within the west, or within the east earlier than dawn.
But at sure occasions of the yr the planet is simple to identify if when and the place to look — and now could be a kind of occasions.
Mercury is the quickest and smallest of the key planets (it is only one.4 occasions wider than our moon). It orbits the sun simply over 4 occasions per yr, however from our shifting perspective on Earth it seems to go round slightly over 3 times. Each yr it makes about three and a half swings into the morning sky and as many into the night — excursions of extremely unequal character due to its eccentric orbit and the various angles from which we view it.
As a rule, the most effective possibilities to make a Mercury sighting within the night come in the course of the spring, and within the morning in the course of the fall. At these occasions, the ecliptic — the imaginary path for the sun, moon and planets in opposition to the background of stars — stands almost perpendicular relative to the western night horizon within the spring, and the morning jap horizon within the fall. As a consequence, after conjunction with the sun, Mercury seems to vault into view over a really brief interval of time.
Take for example, the present apparition.
On Oct. 9, Mercury was at inferior conjunction, which means it was positioned nearly immediately between us and the sun, so it couldn’t be seen. On Oct. 14, Mercury rose about 45 minutes earlier than dawn, however at magnitude +2.5 the speedy little planet was not possible to see within the brightness of daybreak. But on Oct. 17, simply three days later, Mercury was rising 1 hour and 20 minutes earlier than the sun, having brightened to first magnitude, and was seen by way of binoculars above the horizon a bit to the south (proper) of due east about 40 minutes earlier than dawn.
After Oct. 17, Mercury’s general visibility improves quickly. On Saturday morning (Oct. 23) it should rise 2 minutes earlier than morning twilight begins and could have brightened markedly to magnitude -0.4. Among the celebrities, solely Sirius and Canopus will shine brighter.
An unusually favorable best elongation will happen on Monday (Oct. 25), though Mercury is barely 18 levels from the sun. At magnitude -0.6, it should rise in a darkish pre-twilight sky. Through a telescope, the tiny disk of Mercury will seem 57% sunlit. One hour earlier than dawn, will probably be near-impossible to overlook with the unaided eye: a really vibrant yellowish-orange “star” shining alone, low above the east-southeast horizon. The solely object you may get it confused with is the equally hued star Arcturus, which might be at the same altitude however shining above the east-northeast horizon, about 30 levels to the east (left) of Mercury. A clenched fist at arm’s size measures roughly 10 levels in width, so on that morning, Arcturus will glow roughly “three fists” to the left of Mercury.
After that, Mercury steadily turns again towards the sun, but it surely brightens barely as its phase waxes towards full. On Nov. 3, having brightened to magnitude -0.9, look low to the east-northeast horizon about an hour earlier than dawn and you may see Mercury forming a proper triangle with the waning crescent moon and the blue first-magnitude star Spica. The moon will hover 3.5 levels above Mercury, whereas Spica twinkles 4 levels to Mercury’s proper. You may, nonetheless, want your binoculars to spy Spica in opposition to the brightening twilight sky.
Why does it occur?
There are three causes for the speedy reappearance of Mercury after the conjunction of Oct. 9:
- At dawn in autumn, the ecliptic makes a steeper-than-average angle with the horizon for Northern Hemisphere observers.
- Because Mercury passes the ascending node of its orbit on Oct. 15, it’s north of the ecliptic in late October and early November.
- Its orbital pace is close to most, as a result of perihelion (its closest passage to the sun) happens on Oct. 18. Around inferior conjunction, Mercury is far nearer to Earth, and its angular movement relative to the sun is far better than round superior conjunction.
The speedy modifications in Mercury’s brightness and its rising time relative to the sun’s make it doable to foretell inside a day or two when Mercury will final be seen to the unaided eye earlier than it disappears into the intense glow of daybreak twilight. My guess is Nov. 7. Can anybody nonetheless see it after that date?
Joe Rao serves as an teacher and visitor lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers’ Almanac and different publications. Follow us on Twitter @Spacedotcom and on Facebook.