Perihelion, Mercury elongation on January 4
8 comments DownloadTonight is Sunday, Jan 04 2009
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Two astronomical events fall at almost the same time today. First, the Earth is at perihelion – its closest point to the sun for all of 2009. Earth’s perihelion is an event that happens because our orbit around the sun isn’t a perfect circle. For more about this event, see today’s radio show.
Today’s other sky highlight requires you to flip your perspective – thinking of planets not as they are in orbit around the sun – but as they appear in Earth’s sky. Today, the planet Mercury is at its greatest evening elongation. An elongation takes place whenever Mercury (or Venus) lies at its greatest distance from the sun, as seen from Earth. Only these two planets, Mercury and Venus – which orbit the sun inside of Earth’s orbit – ever reach a greatest evening (or morning) elongation. It’s as though they are tied to the sun by a tether, and, at greatest elongation, they’ve reached the end of their tether, stretching as far from the sun as they can – as seen in Earth’s sky.
Today’s greatest elongation provides the best opportunity during this evening apparition of Mercury for catching the solar system’s innermost planet after sunset. Sometimes an elongation carries Mercury relatively far from the sun, but this time Mercury resides less than 20 degrees from the sun (10 degrees is about one fist-width at an arm length away). So, though it’s at its farthest from the sun as seen from Earth now, Mercury still sits close to the horizon at dusk and early evening.
Your window of opportunity for spotting Mercury is from about 45 to 80 minutes after sunset. Find an unobstructed view of your southwest sky and look for this world above the blazing planet Jupiter. If viewing conditions are good, you can see Mercury with the unaided eye. Otherwise, binoculars help out with your Mercury quest. Good luck!
By the way, the best evening elongations of Mercury tend to happen in the spring. So wait a few months, and you’ll have a better chance to see this planet.
This is the Coolest website ever. I have been watching the sky for 47 years, and now I can get an idea of what I am seeing. A big THANK YOU to all involved with this site.
thank you
i live in palm bay florida and for the last week i have noticed a bright star close to the moon and then another that looks like it changes color, could you please inform me what these are and their significance?
Patricia,
The brilliant “star” nearby the crescent moon at dusk and early evening in late December 2008 and early January 2009 was the planet Venus. The brilliant star in the southeast sky at early evening is Sirius, the brightest star in the nighttime sky.
Bruce
I live in the San Francisco Bay Area, last night at around 6pm i looked up at the sky and saw something really bright, i thought it was a star but i looked around and saw no other stars in the sky, which was weird because it was a clear night. The moon was to the right of this bright light. They were really close also.
I got excited and thought it could be a planet but I dont know. This website is pretty cool hopefully you can help me out and tell me what i might have been looking at :)
AC,
On the evening of January 5, the moon was not close to any bright stars or planets. Possibly you saw a weather balloon, but I really don’t know. To investigate the possibilities, I refer you to Larry Session’s article I saw a UFO.
Bruce
It was the evening of January 4, not the 5th
=(
AC,
The moon wasn’t close to any bright stars or planets on January 4 either. Perhaps someone locally knows what that mystery light was. The San Francisco Sidewalk Astronomers may be able to help you. According to the heavens-above satellite visibility site, an iridium flare satellite was visible in San Francisco at about 6:23 p.m. on January 4.
Good luck!
Bruce