Podcaster: David M. F. Chapman
Organization: Astronomy Nova Scotia:
David Chapman’s website:
Description: Venus has appeared high in the western sky throughout the winter. As it nears Inferior Conjunction on March 27, it plunges into the sunset almost 8 degrees North of the Sun, and actually pops up as a Morning Star in the east before it has completely finished its performance as an Evening Star. This may sound impossible, but it is true. For the next several days, Venus can be seen as both a Morning Star and as an Evening Star on the same day. The conditions for this phenomenon recur every 8 years, but the observer needs a low horizon and clear skies to see it, as Venus is very close to the Sun.
Bio: David Chapman has been an amateur astronomer for nearly 50 years, ever since his father showed him the constellation Orion in the winter sky. He has observed all the Messier Objects, two total solar eclipses, and 2 great comets. He joined the Royal Astronomical Society of Canada in 1983 and is now a Life Member. He was President of the RASC Halifax Centre in Nova Scotia for 2 years and now looks after their web page. He has written scripts for the NPR show StarDate and wrote a series of 50 columns for the Journal of the RASC. He has been a Contributing Editor of the Journal and is currently an Assistant Editor. He is actively involved in local IYA activities in 2009.
Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Loretta Van Orden on behalf of her favorite amateur astronomer, Penni Nelson, in Post Falls, Idaho.
Hello, this is Dave Chapman from Astronomy Nova Scotia in Canada. Today I would like to tell you about this week’s conjunction of the planet Venus with the Sun, our star. This conjunction is a rare event with an observing challenge for amateur astronomers, but all you will need is pair of binoculars and clear skies.
If you were outside on any clear evening this past winter, you must have noticed the planet Venus, brilliant and high in the western sky after sunset. Apart from the Sun and the Moon, Venus is the brightest object in the sky, brighter than any star and even brighter than the giant planet Jupiter. Why is Venus so bright? Well, for a start Venus is perpetually shrouded in cloud, making it the most reflective planet in the Solar System.
Secondly, Venus is only the second planet from the Sun, so the sunlight it receives is more intense than every planet other then Mercury. Finally, Venus seems bright to Earthlings because it is the closest planet to us: our nearest neighbor, so to speak.
The orbit of Venus around the Sun is inside the orbit of Earth, and along its orbit Venus travels faster than the Earth, so Venus is continually catching up with Earth and passing us by. Imagine that Venus and Earth are running on a circular racetrack with fleet-footed Venus on the inside track and pokey Earth dragging its heels on the outside track: Venus would lap the Earth every 584 days, that is, a little over 19 months. This race may not seem fair, but this imaginary track meet is governed by Kepler’s Laws of Orbital Motion, not the International Olympic Committee!
Because Venus is on the inside track, it never appears more than 47 degrees away from the Sun. An angle of 47 degrees is about halfway from the horizon to directly overhead. When Venus appears East of the Sun, we see Venus in the evening sky after sunset and we call it the Evening Star; when it appears West of the Sun, we see Venus in the morning sky before sunrise and we call it the Morning Star. These names may seem old-fashioned to us today, but in ancient times, Egyptian and early Greek astronomers actually believed these two apparitions of Venus to be two separate bodies.
In 2009, Venus in its role as Evening Star reached its maximum separation from the Sun on the 14th of January. As Venus catches up with Earth, it appears to be slowly moving closer to the Sun in the sky. Every night at sunset, Venus is closer to the horizon than the night before. As Venus approaches us, its apparent size is growing, but we only see that part of the planet that the Sun illuminates, so Venus now appears as a slender crescent, like a tiny crescent Moon. You can observe the crescent of Venus in a small telescope on a sturdy mount. You may even use binoculars, but you will need to hold them steady to see the crescent clearly.
Viewed at sunset every evening, Venus has left its lofty position in the winter evening sky and lately has dropped very close to the horizon near the sunset position. In fact, in a few days it will pass between the Earth and the Sun as moves through its point of closest approach to Earth. This event is called an Inferior Conjunction. Having spent 9 months as an Evening Star at dusk, Venus will soon begin its appearance as a Morning Star in the dawn’s early light.
The way I have described this transition, you might think that Venus will pass directly in front of the Sun. Although this alignment sometimes takes place, it does not happen at every Inferior Conjunction. When it does, it is called a Transit of Venus. The next Transit of Venus takes place three years from now on the 6th of June, 2012. The following Transit of Venus does not take place for over a century.
Why are these transits so rare? Because the orbit of Venus is tilted by 3 degrees, at most conjunctions Venus passes either to the north or to the south of the Sun. For this week’s conjunction on the 27th of March, the orbital inclination of Venus combined with its close approach to Earth has Venus passing almost 8 degrees north of the Sun. This angle is a little less than the width of a tightly clenched fist held at arm’s length.
The wide separation of Venus and the Sun at this conjunction provides an interesting opportunity for amateur astronomers at mid-northern latitudes: starting now, and for several days, Venus appears an Evening Star and as a Morning Star on the same day! In other words, Venus will set just AFTER sunset and rise just BEFORE sunrise. At these times, the sky will not be very dark and it may be necessary to use binoculars to find Venus, which admittedly will not appear very starlike on this occasion. You will need to find a viewing spot with a low horizon and hope for very clear skies to spot Venus this week. Another tip: Venus will set a little to the RIGHT of the Sun’s position at sunset and rise a little to the LEFT of the Sun’s position at sunrise.
If you plan to look for Venus this week, please heed the following safety caution: DO NOT USE YOUR BINOCULARS TO LOOK FOR VENUS WHEN THE SUN IS ABOVE THE HORIZON. You could accidentally expose your eyes to the intensified light from the Sun and damage your vision. In the evening, make sure the Sun has set before using your binoculars; in the morning, start your search well before sunrise and stop the instant the Sun rises. If you attempt this observation, and have good observing conditions, you will be rewarded by a wonderful, rare sight: the large, slender crescent of Venus at its closest approach to Earth, making its way past the Sun from the evening sky into the morning sky.
Returning to the circumstances of this conjunction, you might be wondering: how often does this happen? It turns out that 8 Earth years is almost exactly 13 Venusian years, so Venus and Earth occupy almost the same positions in their orbits every 8 years. There are reports of amateur astronomers spotting Venus at Inferior Conjunction in 1977. At the next opportunity, I myself observed Venus as a Morning Star and Evening Star on the 27th of March, 1985. Unfortunately, for the following opportunities in 1993 and 2001, the Nova Scotian weather did not cooperate with me. Perhaps the International Year of Astronomy 2009 will bring me better luck!
This is Dave Chapman from Astronomy Nova Scotia wishing you clear skies and pleasant observing.
Astronomy Nova Scotia is a collective of amateur astronomers, professional astronomers, and science educators in Halifax, Nova Scotia, Canada, We have joined together to celebrate and promote the International Year of Astronomy 2009. For more information, please visit our website AstronomyNovaScotia-DOT-ca.
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365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.