Monday, November 24, 2008

jupiter and venus race to december's horizon

if you have been able to see the early evening skies for the last several months, you have almost certainly noticed two very bright "stars" sinking towards the horizon: venus and jupiter.

venus sets first in the southwest and appears 2 magnitudes brighter than jupiter, which hangs higher in the sky (about 7° apart nov 23rd). watch these bright planets over the next week, as they move together in night sky. jupiter is moving more quickly along the ecliptic, and therefore appears to catch up to venus in the southwestern night sky, as if it wants to win the race to set earlier! by november 30th, the planets are only 2° apart, when the party is invigorated by a wee crescent moon, that passes near the planetary conjunction as it moves the opposite direction in the sky by 12° each night.

jupiter then continues its descent and slowly falls a bit westerly each night. the bright planet sets earlier and earlier so that it eventually catches up with the sun's position in our night sky! jupiter physically passes behind the sun next january 24th as it says goodnight to our night sky. when the planet moves around the far side of the sun, it begins to lead the sun along the ecliptic, and starts to greet us in our morning sky, as it rises just before the sun rises to bring the day.

UPDATE: a comment from chuck convinced me i needed to clear up issues about objects moving around in the night sky!

every night, all objects rise in the east and set in the west, just like the sun, due to earth's daily rotation. over time (many nights or weeks or months), the planets move more quickly with respect to the rest of the stars because they are closer to us in space (this was how we originally identified planets).

while each night venus and jupiter rise in our eastern skies and set in the west, over time they continue along their own orbits around the sun which causes them to move along the ecliptic defined in our sky. currently, jupiter is moving westward along the ecliptic causing it to set a bit earlier each night, while venus is moving, much more slowly, in the other direction, along the ecliptic. so venus isnt actually racing jupiter to set earlier, its very slowly setting later and later. sorry for the confusion, and thanks chuck!


lalamandala said...

i notice you've started using the word "wee" :) one of my faves! and i have been watching these planets. sometimes they're the only things i can see in our unfortunately too bright ohio night sky. at any rate they make me think of you and that's always a happy thought!

C W Magee said...

Isn't Venus still going up?

Unknown said...

youre right, chuck, i think i wasnt clear on this point. i'll add a bit of explanation to the post...


Anonymous said...

I've been watching the Phoenix skies for the past months and have been noticing these 2 bright 'stars' at night. I live on a hill and at night, the sky is so beautiful, I can see all the stars. Last night, I was mesmerized by the brightness of these 2 stars. I do not own a telescope, I can only imagine what they look like closer. :-)

Arizona sky watcher

Anonymous said...

When will we see this again? Will it be many years from now?