Image credit: NASA/LMSAL
“…image shows Venus on the eastern limb of the Sun. The faint ring around the planet comes from the scattering of its atmosphere, which allows some sunlight to show around the edge of the otherwise dark planetary disk. The faint glow on the disk is an effect of the TRACE telescope.”
To recap an earlier post, a transit is when a smaller astronomical body passes in front of another. We have a rare transit coming up next week, Venus transiting in front of the Sun. There are only two planetary transits from Earth’s viewpoint, Venus and Mercury. Because Mercury orbits the Sun at a faster rate than Venus, we can expect 12-13 transits per century. Venus, on the other hand, transits in pairs 8 years apart in intervals of ~105.5 or 121.5 years (see chart). The last solar transit of Venus was June 8th, 2004. The next one in the pair is June 5/6, 2012. The previous transits were December 1874 and December 1882. After the upcoming transit, the next pairing is slated for December 2117 and December 2125.
Kepler had predicted a transit for 1631 but didn’t predict another for 1639. Jeremiah Horrocks reviewed Kepler’s calculations and determined the possibility of a second transit, which he observed by means of solar projection in November 24, 1639.
In the 1700s, Edmond Halley used the Venus transits as a means of measuring distances in the solar system using Kepler’s third law that states the orbital period squared is proportional to the distance cubed. Halley organized observations at several global observing sites for the 1761 and 1769 Venus transits, realizing that each observer’s line of site would have a slightly different placement on the Sun. By timing how long it would take Venus to transit, he could determine how far away the Sun was from Earth. What’s mind boggling is that the resulting astronomical unit (AU) from that experiment was between 149.67-156.11 million kilometers and we now have a more accurate AU of 149.60 million kilometers.
To help with understanding all this, here are a few definitions/figures that are useful to know:
- A meter (m) is “the length of the path traveled by light in vacuum during a time interval of 1 ⁄ 299,792,458 of a second.” (Bureau International des Poids et Mesures)
- Light travels 299,792,458 m per second (186,282 miles/second) and is known as “speed of light” or simply “lightspeed”.
- Distance from the Earth to the Sun is called an astronomical unit (AU), which = 149.60 million kilometers (92.956 million miles). The speed of light from Sun to Earth (1 AU) = 8.3 min.
Using the ephemeris from my Starry Night program, the AU of Venus from my observing location during the transit from 1706 UT – 2032 UT (sunset) will be 0.2887 AU and from the Sun will be 0.7260 AU.
Screenshot from Starry Night
Another handy tool is a freeware program called WinJUPOS 9.1.8 – Database for Object Positions on Planets and the Sun by Grischa Hahn.
Screenshot from WinJUPOS 9.1.18 – Database for Object Positions on Venus
Here’s an excellent link taken from NASA’s site for the 2012 Transit of Venus that will be published in Observer’s Handbook 2012, Royal Astronomical Society of Canada. The link provides valuable information on how and when to observe the upcoming transit and from which locations. It also provides the history of the Venus transits and calculator tools for geographic positions. Also see Transit of Venus 2004 as taken from NASA’s site as Published in Observer’s Handbook 2004, Royal Astronomical Society of Canada.
From ALPO: ALPO Astronomy – “Local Circumstances for the Transit of Venus , June 5-6 2012″
Arvind Paranjpye wrote a helpful tutorial on Sketching the transit of Venus 8th June 2004. Peter Grego will also have an article in Astronomy Now magazine June 2012 issue for sketching the transit.
Good luck with your upcoming Venus Transit observations! REMEMBER SOLAR SAFETY: never look at the Sun directly without the use of a proper solar filter. Here are “Six Ways to See the Transit” by Chuck Bueter.
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