2012 was a good year to look at the sun. Here in Northern California we had the chance to observe a couple of rare solar phenomena. The first, on May 20th, was an annular eclipse, the centerline of which crossed the landscape just south of the Mount Shasta region. The sky faded to a deep twilight blue, dimmest at 6:30 p.m., and the dappled shadows resolved to a pattern of over-lapping rings on the ground beneath the trees (the pin-hole camera effect). The view through welder’s glass showed an ever-bigger bite being taken from the sun as the moon progressed across its face. But the horns of the final crescent only kept growing longer, not shorter – total coverage was never achieved. The horns merged at max eclipse, turning the sun into a thin circle, as dark in the center as outside the outer edge. 95% of the solar diameter was blocked by the moon, yet it was still far too bright to look at without heavily filtering the light.
Two weeks earlier, we had witnessed the rise of the “supermoon” at sunset. This was the biggest the moon appears, seen at its brightest and closest as a coincidence of the perigee in the elliptical lunar orbit and the full lunar phase. Now, on the opposite side of its orbital track, the moon was at its farthest away and its smallest, at apogee. During most total eclipses the sun’s corona streams out across a dark sky, and pink prominences reach up from the dark edge of the moon. But at lunar apogee, because the moon appeared smaller, we saw the annular eclipse instead this time – incomplete coverage, producing the gold ring of fire.
Annular eclipses are relatively new phenomena. For most of Earth’s history, the moon has been in a closer orbit. Passage of the moon across the face of the sun has brought nightfall to the middle of the day, and lasted more than just a few minutes. But the moon is continually receding from the Earth. Just now, we happen to live in an era when the visible diameter of the moon has diminished to just equal that of the much larger, much farther away sun. But as time draws on, if we manage to last as a race long enough to see it, the moon will continue to spiral away, and all total eclipses will be annular.
Venus came out to shine through the darkened midafternoon sky that day in May at the peak of the eclipse. It was riding down the descending node of its orbit – approaching its closest pass by Earth, plunging toward the western horizon. The planet had been the biggest, brightest star-point to be seen in the night sky for the previous ten months, but two weeks and two days after its daylight appearance on May 20, Venus’s role was reversed.
Venus is usually seen as a star – its circular outline hidden by circumferential spikes that actually arise in our own corneas when we focus on point sources of light – like stars. But on June 5th bright Venus was transformed into the darkest sight in the sky – into a little black circle moving across the face of the sun. Venus moved directly across the solar disk, as seen from half of the surface of the Earth. These transits come in pairs 8 years apart, but in western North America we had missed the 2004 event – it was blocked from sight by the interposition of the Earth (e.g., for west coast observers, it happened at night).
Viewed at the same afternoon angle that it had been seen as a day star half a month earlier, Venus became a tiny black spot on the solar disk on June 5th, just visible by eye (through the dark eclipse filter). It was in transit all afternoon, through sunset. We saw the syzygy from Northern California where two planets – the Earth at its horizon and the entire sphere of Venus – were both silhouettes on the face of the setting sun. The next time that will happen will be next century.