Planning sightlines to the sun

One aspect of the transit of Venus that people eagerly wanted to share with me after the 2004 event was their seeing Venus when the sun was near the horizon. Some people suggested an equivalent experience to the “moon illusion,” in which the full moon seems larger near the horizon than when it is high overhead, when in fact their is no change in its angular dimension over several hours. To those witnesses, even though Venus is only 1/30th the diameter of the sun–about one arcminute across, or near the limit of the human eye to detect–when the sun was near the horizon both the sun and Venus seemed bigger. This was their unwitting confession, for it tells me they were looking at the sun near the horizon with their naked eyes. While I cannot condone looking at the sun without proper eye protection, I have to concede everyone has looked at a sunset or sunrise sans eyewear. When one looks through a solar filter, the surroundings will not be visible in the blackness, so no illusion effect would occur.

A light post obstructs a view of the sun.

If you are thinking of hosting a public event to celebrate the 2012 transit of Venus, now is a good time to begin some basic planning. We will have had eight years since the last transit of Venus to prepare, yet there suddenly won’t seem like enough time. Since seeing the sun and Venus near the horizon is such a highlight, as reported in the past, you better have an unobstructed sightline to the sun in 2012. You don’t want some tree or pole to interfere with your view of the sun. This week is your last chance to preview where the sun will be at either sunset on June 5 or sunrise on June 6, 2012. Since the June solstice, the sun has been retreating to the south, but around Thursday, July 7, the sun will pass the same horizon location as it does every June 5-6.

That compass bearing to the sun, among 360 degrees, is called its azimuth. You can find the sun’s azimuth during key moments of the transit of Venus at But if you literally want to see where the sun will be that evening, and not be surprised by any intervening structures, traffic, or impediments, you can visually prepare where you should set up in June 2012 by watching the sun set this week. More ideas for planning a community celebration are at I welcome your input and suggestions.

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