Thursday, September 30, 2010

Friday photo (special Thursday edition)

Mercury and Moonrise over Absaroka Mountains,
Yellowstone National Park. September 30, 1997.

(Be sure to click on the image for the enlarged version)

My 35mm slides are dated only to month and year, but according to Stellarium, this conjunction of Mercury and the new moon occurred on September 30, 1997. I didn't know it just yet, but that was the day my brother's wife gave birth to their first daughter. In fact, I never connected that event to this photo until I was preparing to post it a couple months ago. So here's wishing a Happy Birthday to Olivia, whose birth 13 years ago was announced with unusual and beautiful signs in the heavens.

And Mercury is an unusual sight, even if the moon weren't so close by*. Most folks will never glimpse it -- not knowingly anyway. Being closer to the sun than Earth, it can only appear at sunrise or sunset, like Venus does, but it's so much closer to the sun that it's usually much harder to spot. It needs to be well out to the side, as seen from Earth, or it's overwhelmed in the sun's glare. Also, if its path around the sun - the ecliptic - is parallel to Earth's horizon, then it will rise at the same time as the sun and again the sky is too light. The best viewing is in the early spring or early autumn, when the ecliptic is more vertical to our horizon. This allows Mercury to rise higher before sunrise, or set later after sunset, when the sky is slightly dimmer, like so:

This is the arrangement on 9/30/1997, so you can see that Mercury was in a good position, although not the best. However, I wanted the moon in the photo, and the moon changes position every day; it was the 30th or nothing.

t also happens that Mercury is most visible when it's on the opposite side of the sun from Earth, even though it's farther away then. Otherwise, even though it's closer, it presents mostly its dark side to the world -- much like the moon in the photo, or a Goth chick.

* "Close by," of course, as viewed from Earth. The planet was actually 500 times farther away than the moon was.

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