Monday, June 26, 2006

Remember Manhattanhenge?

Tom Clabough, one of the Central Park "Star Guys" who frequently set up telescopes at the top of the Great Lawn and generously share their sightings and knowledge with passersby, has sent in another clarification of the Manhattanhenge effect. I thought understanding this was pretty hopeless for me, but this time I felt a glimmer of
getting it.

[IMAGE: The apparent path of the Sun across the sky]
Hi Marie, I've just come across this excellent graphic depicting the Sun's seasonal migration north and south of due east and due west. The red line represents the Sun's position in the sky during summer solstice. The green, winter solstice. And the blue both the spring and fall equinoxes. Note how in summer the Sun rises and sets north of the east-west line. And in winter, south of the east-west line. Also note the inclination of the Sun during these times. i.e. high in the summer, low in the winter. During spring and fall at an intermediate inclination. All of this movement of the ecliptic is due to the 23-1/2 degree tilt of the Earth's axis. In fact, this migration of the eclipitic is actually a daily one. One rotation of the Earth, one complete cycle of ecliptic migration. The one thing that changes is the Sun's position on the ecliptic. As we seasonal orbit the Sun, it appears to move through the 12 Zodiacal constellations, one per month. This happens because the Sun is much closer to earth than are the background stars. As we orbit around the Sun, our line of sight with respect to these very distant constellations, changes.

Recall that the ~ "north-south" axis of Manhattan island (the street grid) is actually skewed about 29 degrees to the east from true north. If you'd like, you can copy the above image on a piece of paper and draw in a line which is at an angle of ~29 degrees to the east of the true north/south line. This will then represent the alignment of Manhattan island. To perhaps visualize this a bit better, try imagining that you are standing along the very bottom of the south path of the Great lawn such that you are facing directly uptown. From this position, true north (and the north star) will be off to your left. For this reason, "Manhattan-henge" occurs approximately 22 days before summer solstice, and again, ~22 days after summer solstice. At both the spring and fall equinoxes, the Sun rises and sets exactly true east and true west.

I hope this clarifies what I sent you earlier. Take care for now. See you in da park. :)


PS- the link to the NASA site where I found this is at the top. There's lots more info there. They do not mention the Manhattan-henge effect, however.
[IMAGE: The apparent path of the Sun across the sky]