I don’t remember how I first heard about it — perhaps through my Twitter feed — but a couple of weeks ago I resolved to watch the Transit of Venus, though I wasn’t sure how to go about it.  Then a few days ago I saw a news brief on the UMass website announcing that there would be a viewing of the transit at the UMass Sunwheel, sponsored by the Department of Astronomy.  I’m there, I said to myself.

All day today I watched the sky anxiously.  It remained drizzly and cloudy, so I began to think that we were out of luck.  But the rain had stopped by 5 pm when I usually leave work, so I drove out to the Sunwheel, which is just behind the stadium.  At 5:30, Professor Stephen Schneider spoke to the crowd about the Sunwheel itself, and asked us to hang around to see if the clouds would clear.

A number of ancient cultures built stone structures to serve calendrical functions, Professor Schneider explained, because back in those days, agricultural societies depended on the sun’s position to indicate to them when to begin planting crops.  Now about ten years old, the UMass Sunwheel was designed to demonstrate how these stone calendars worked.  The first tall standing stones to be placed in the Sunwheel indicate the cardinal directions: north, south, east, west.  On the spring and fall equinoxes, the sun rises over the East stone and sets over the West.  In the night sky, Polaris appears directly above the North stone; its angle above the horizon should be around 42 degrees, because Amherst is at latitude 42 degrees.  Due to our latitude, at the equinoxes, the sun rises no higher in the sky than 48 degrees.  Because the Earth tilts 23.5 degrees on its axis, the sun rises to only 24.5 degrees at the winter solstice, and climbs to 71.5 degrees at the summer solstice.  In the northern hemisphere, at noon on the solstices, shadows fall to the north.

Now then: Venus.  It is our sister planet, in a sense, and also tilts on its axis as it orbits the sun.  However, Earth’s orbit and Venus’ orbit are not tilted at the same angle, so the alignment of their orbits happens twice in eight years, every hundred years or so (the previous transit occurred in 2004).  In the nineteenth century, transits occurred in 1874 and 1882, and the next won’t be until 2117!  Though the transit is not at all spectacular like an eclipse, its rarity makes it a special astronomical event.

Professor Schneider reminded us that as recently as the eighteenth century, the size of the solar system could not be accurately calculated:  people didn’t really know how big the sun or the planets were, or how far away they were from each other.  In 1716, the astronomer Edmund Halley suggested that more accurate numbers could be obtained by measuring the transit of Venus; that is, from widely-spaced locations on Earth, observers could time the transit across the sun and by triangulation derive the distances to Venus and to the sun.  In 1761, the Americans Mason and Dixon, they of Mason-Dixon Line fame, travelled to South Africa to participate in the collaborative scientific effort to take accurate measurements.  The 1769 transit was also successfully observed from many locations worldwide.

By 5:50, the western sky was clearing, and by 6, the sun was actually visible!  The crowd had swelled to over 50 people by then, and the excitement was palpable.  The two telescopes, both with filters that enabled safe viewing, were set up and trained on the sun.  At 6:04 I started looking through the special glasses that were provided for us, but I could only see the sun as a perfectly round orange circle.  I knew that Venus would appear as a very small black dot, so I figured that my naked eyes weren’t good enough to see it.  So I got in line for one of the telescopes, and after only a few minutes, it was my turn.  So I looked through the eyepiece, and there it was! a little black dot at the left and bottom of the solar disk.

What a thrill it was to have this once-in-a-lifetime experience.