Archive for March, 2012
Elongation, in astronomy, refers to the angle between the Sun and a planet as viewed from Earth. We’ve been talking about certain planets and the Moon being at opposition, meaning they appear to us to be opposite the Sun, or that their elongation is 180 degrees. This is possible for the Moon because it orbits the Earth. It is also possible for Mars, Jupiter, and Saturn (and the other outer planets) because they are farther from the Sun than the Earth. Planets closer to the Sun like Venus and Mercury cannot be at opposition. Their greatest elongation is substantially smaller due to our perspective. Today Venus reaches its greatest elongation: 46 degrees. This angle will start to shrink in the coming weeks.
In the telescope, Venus appears half lit like the first quarter Moon. The lit side will shrink to a crescent as it continues to move in its orbit to a position between us and the Sun. In early June a very rare and special event will occur: a transit of Venus across the face of the Sun. Since the following transit won’t occur until December 2117, we will be talking about the 2012 transit a lot more as it approaches.
The waxing crescent Moon will be near Jupiter and Venus in the western sky at sunset the next few nights. This show has been repeating approximately once a month for several months now. The Moon traces nearly the same path across the sky from month to month and passes by several planets as it does so – it will pass by Mars in a few days. This is a testament to the near perfect flatness of our Solar System. Jupiter is now nearly right below Venus and they keep pulling farther apart. Soon Jupiter will dip below the horizon and we won’t see it again in the evening until late fall. But, don’t fret – Saturn is approaching opposition and we should have great views through the summer.
You wouldn’t know it from the weather in the Midwest recently, but the first day of spring for the northern hemisphere is March 20. This is the March equinox – also called the vernal or spring equinox, but we must remember that it also marks the first day of fall for the southern hemisphere. Equinox comes from the Latin for “equal night” and, indeed we have approximately the same number of daylight and night hours this time of year. We can give an exact time for this event (12:14 AM CST) because this is the moment the Sun is at the intersection of its path on the celestial sphere with the celestial equator. On December 21 we talked about the Sun being as far south of the celestial equator as it could be and its apparent path has been taking it farther north since. On Tuesday morning it crosses the celestial equator and continues north. In June we’ll talk about the northern solstice.
Full Moon was on March 8. The Moon was very prominent in the east right around sunset. It is now in the waning gibbous phase and will be rising later and later in the evening. Jupiter and Venus are moving closer and closer together in the west and appear around twilight. Their closest apparent approach will be on March 13.
The sky was very different just a couple weeks ago. I took this picture of the Venus-Jupiter-crescent Moon conjunction on February 25:
The Moon is very close to Venus here and Jupiter is the faint point in the upper left corner. Note that the Moon appeared in the west-southwest here and that Jupiter and Venus appeared much farther apart than they do currently. It was slightly hazy that night which is why Jupiter was so dim. Now here’s the view the very next night:
The Moon shows the greatest change in position when viewed around the same time each night because it is so close to us. The other planets will show a similar shift but it’s less dramatic as they are farther away. Watch Venus and Jupiter over the next few days and this shift will become very apparent. Thanks again to Kate Davis for cropping the photos!
The planet Saturn is famous for its extensive ring system, visible even through small telescopes. Galileo was the first to observe Saturn’s rings in 1610, but they were not described as such until the mid 1650s by Christiaan Huygens. That Jupiter, Uranus, and Neptune also had ring systems (albeit considerably less spectacular) was not known until long after the rings of Saturn were observed.
It was on this date (March 10) in 1977 that the 2nd planetary ring system in the Solar System was discovered. Astronomers hoping to study the atmosphere of Uranus by observing it pass in front of a star noted that the star dimmed several times before it disappeared behind Uranus and then again after it reappeared. The photo below shown courtesy of NASA and the ESA and obtained by the Hubble Space Telescope in 2005, shows the narrow set of rings that caused the star to dim in 1977.
Mars was at opposition on March 3 and was closest to the Earth for 2012 on March 5. You have probably noticed it as the bright reddish “star” rising in the east at twilight. Good views of Mars will continue through the spring, but it will slowly dim as we are now moving farther apart again. The next closest approach to Earth won’t be until April 2014 so enjoy the show!
If you have clear skies in next few evenings be sure to try and catch a special event. At approximately 45 minutes after sunset you should be able to view the 6 brightest objects in the night sky. The Moon (1st brightest) is currently above the south-southeast horizon and just entered its waxing gibbous phase. We’ve been talking about the planets Venus and Jupiter (2nd and 3rd brightest respectively) for some time now. They are moving closer together and shifting towards the western horizon. The star Sirius is the 4th brightest object. You took my advice and have gone out and found Orion, right? Trace a line through Orion’s belt towards the horizon and you’ll easily see bright Sirius. The planet Mars (5th brightest) is rising in the east. You can’t miss its reddish appearance. The planet Mercury (6th brightest) is perhaps the most challenging as it appears just above the western horizon. Trace a line through Venus and Jupiter towards the horizon and you should spot Mercury. Don’t wait too long though, it dips below the horizon relatively quickly.
Those of you in the southern US are actually able to view the 7 brightest objects! You can add the star Canopus to the list, just above your southern horizon. It never rises above the horizon for us further north.
This event won’t happen again for decades so enjoy the show!