Bird’s eye view of the inner solar system (Mercury, Venus, Earth and Mars), when Mercury is at inferior conjunction on September 30, 2015. From this vantage point, all planets circle the sun in a counter-clockwise direction. The blue represents the part of the planetary orbit to the north of the ecliptic – Earth’s orbital plane.
Bird’s eye view of the inner solar system (Mercury, Venus, Earth and Mars), when Mercury reaches it greatest elongation from the sun on October 16, 2015. From this vantage point, all planets circle the sun in a counter-clockwise direction. Image credit: Solar System Live
On September 30, 2015, Mercury – the sun’s innermost planet – passes in between the sun and Earth. In the parlance of astronomers, Mercury is said to be at inferior conjunction on this date. Mercury, named for a fleet-footed messenger god, speeds around the sun rapidly, taking only 88 days (less than three earthly months) to travel around the sun once. So it won’t surprise you to learn that Mercury was last at inferior conjunction – last between us and the sun – on May 30, 2015. It’ll reach inferior conjunction again, gaining yet another lap on Earth in the endless race of the planets, on January 14, 2016.
The image at the top of this post shows Mercury (innermost planet) and Earth (3rd planet from the sun) line up with the sun on September 30, 2015.
Of course, we can’t see Mercury in Earth’s sky at inferior conjunction. At such times, this inner world is totally lost in the sun’s glare. But this inferior conjunction marks an important milestone for Mercury as viewed in Earth’s sky. It marks the moment Mercury transitions out of Earth’s evening sky and into its morning sky.
What’s more, Mercury will reach perihelion – closest point to the sun in its orbit – on October 12, 2015. Although Mercury is always the swiftest-moving planet of the solar system, Mercury moves most quickly at perihelion and most slowly at aphelion – its farthest point from the sun. In fact, Mercury moves about 20 kilometers (12 miles) per second faster at perihelion than it does at aphelion. At perihelion, Mercury speeds around the sun at nearly 60 kilometers (37 miles) per second – twice as fast as our planet Earth’s orbital velocity.
For that reason, Mercury will swiftly climb upward into the morning sky during the first two weeks of October, 2015, to reach its greatest western (morning) elongation on October 16, 2015.
At its greatest western elongation, Mercury will swing out to its greatest angular distance from the sun, to showcase itself as a morning “star” in the eastern dawn sky.
If you live in the Northern Hemisphere, there’s a reasonably good chance of catching Mercury in the morning sky as soon as October 10 or 11. Think photo opportunity. If you’re blessed with an unobstructed eastern horizon and a clear sky, you may spot the whisker-thin moon and Mercury over the horizon about one hour before sunrise.
Circle October 10 and 11 on your calendar. That’s when you have the opportunity to view the waning crescent moon and Mercury coupling up in the eastern sky at morning dawn. May you be blessed with clear skies!
Bottom line: On September 30, 2015, Mercury is at inferior conjunction – between the Earth and sun – and lost in the sun’s glare. Starting in the second week of October, we expect some keen-eyed observers to spot Mercury in the east before dawn.
from EarthSky http://ift.tt/1FbOowa
Bird’s eye view of the inner solar system (Mercury, Venus, Earth and Mars), when Mercury is at inferior conjunction on September 30, 2015. From this vantage point, all planets circle the sun in a counter-clockwise direction. The blue represents the part of the planetary orbit to the north of the ecliptic – Earth’s orbital plane.
Bird’s eye view of the inner solar system (Mercury, Venus, Earth and Mars), when Mercury reaches it greatest elongation from the sun on October 16, 2015. From this vantage point, all planets circle the sun in a counter-clockwise direction. Image credit: Solar System Live
On September 30, 2015, Mercury – the sun’s innermost planet – passes in between the sun and Earth. In the parlance of astronomers, Mercury is said to be at inferior conjunction on this date. Mercury, named for a fleet-footed messenger god, speeds around the sun rapidly, taking only 88 days (less than three earthly months) to travel around the sun once. So it won’t surprise you to learn that Mercury was last at inferior conjunction – last between us and the sun – on May 30, 2015. It’ll reach inferior conjunction again, gaining yet another lap on Earth in the endless race of the planets, on January 14, 2016.
The image at the top of this post shows Mercury (innermost planet) and Earth (3rd planet from the sun) line up with the sun on September 30, 2015.
Of course, we can’t see Mercury in Earth’s sky at inferior conjunction. At such times, this inner world is totally lost in the sun’s glare. But this inferior conjunction marks an important milestone for Mercury as viewed in Earth’s sky. It marks the moment Mercury transitions out of Earth’s evening sky and into its morning sky.
What’s more, Mercury will reach perihelion – closest point to the sun in its orbit – on October 12, 2015. Although Mercury is always the swiftest-moving planet of the solar system, Mercury moves most quickly at perihelion and most slowly at aphelion – its farthest point from the sun. In fact, Mercury moves about 20 kilometers (12 miles) per second faster at perihelion than it does at aphelion. At perihelion, Mercury speeds around the sun at nearly 60 kilometers (37 miles) per second – twice as fast as our planet Earth’s orbital velocity.
For that reason, Mercury will swiftly climb upward into the morning sky during the first two weeks of October, 2015, to reach its greatest western (morning) elongation on October 16, 2015.
At its greatest western elongation, Mercury will swing out to its greatest angular distance from the sun, to showcase itself as a morning “star” in the eastern dawn sky.
If you live in the Northern Hemisphere, there’s a reasonably good chance of catching Mercury in the morning sky as soon as October 10 or 11. Think photo opportunity. If you’re blessed with an unobstructed eastern horizon and a clear sky, you may spot the whisker-thin moon and Mercury over the horizon about one hour before sunrise.
Circle October 10 and 11 on your calendar. That’s when you have the opportunity to view the waning crescent moon and Mercury coupling up in the eastern sky at morning dawn. May you be blessed with clear skies!
Bottom line: On September 30, 2015, Mercury is at inferior conjunction – between the Earth and sun – and lost in the sun’s glare. Starting in the second week of October, we expect some keen-eyed observers to spot Mercury in the east before dawn.
from EarthSky http://ift.tt/1FbOowa
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