Here a Henge, There a Henge: Astronomy Fun on a Street Near You

Manhattanhenge, by EffingBoring via Flickr

Invited Guest Post by Evelyn Lamb (@evelynjlamb)

Tomorrow (Wednesday) the setting sun will align with Manhattan?s street grid to produce a striking phenomenon dubbed ?Manhattanhenge.? Taking its name from the more famous Stonehenge in England, where the sun rises over the prominent Heel Stone on the summer solstice, Manhattanhenge happens twice a year, once about three weeks before the summer solstice (May 29th and 30th this year) and once about three weeks after (this Wednesday and Thursday).

Neil DeGrasse Tyson, director of the Hayden Planetarium at the city?s American Museum of Natural History, is credited with ?discovering? the phenomenon, which has grown in popularity since he published a photo of the setting sun from a vantage point looking west down 34th Street about a decade ago. An astrophysicist in one of the most densely populated places in the country, Tyson has spent a lot of time finding ways to connect New Yorkers to the astronomical world, and he is glad that Manhattanhenge has resonated with so many people.

Despite what some New Yorkers may think, the sun rises and sets all over the world, so in theory any city laid out on a grid, or even an individual straight road, might have its own ?henge.? How can we play along at home, and find out when we might see our own neighborhood henge event?

First, here?s a little background on henges. Why do they occur, and why twice a year? Contrary to popular belief, the sun doesn?t quite rise in the east and set in the west daily. Away from the equator, the sun only rises at due east and sets at due west twice a year: at the equinoxes in March and September. After the March equinox that heralds spring in the Northern Hemisphere (autumn in the Southern), the azimuths, or directional bearings, of sunrise and sunset move north, then stop at the June solstice and head back south, passing through the September equinox before hitting the December solstice and reversing direction once more.

The farther north a Northern Hemisphere location is?and the farther south a Southern Hemisphere location is?the more offset a grid can be and still line up with the sun at some point in the year. Manhattan?s grid, for example, is about 29 degrees off from the geographical grid, not quite as tilted as the angle of sunset at the summer solstice, so the sun lines up with the grid a few weeks away from the summer solstice on either side.

Finding the date when the sun will rise or set directly along the line of your street involves calculating the azimuth of both the sun and the road (or other candidate landmarks) in question. The azimuth is measured in degrees clockwise from north, so due east is 90 degrees, south is 180 and west is 270. Manhattan?s tilt of 29 degrees corresponds to an azimuth of 109 to the east and 299 to the west.

To find your own street?s henge azimuth, you can use the interactive online compass at http://googlecompass.com/ and search for your address, or whatever city or landmark you are interested in. When you have zoomed in to a comfortable level, click on ?Show Compass? at the bottom left. A compass will appear on the map. You can drag this compass to whatever location you want on the screen and then rotate it around until the arrow points in the direction of the street in question. To find a sunrise henge, point the arrow toward the east; for a sunset, point the arrow to the west. The degree of the angle the arrow makes with true north will appear in the top right corner.

A number of factors influence the direction of sunrise and sunset in a particular place at a particular time. Latitude and time of year are chief, but the ellipticity of Earth?s orbit and the fact that the planet is not quite spherical must also be taken into account. Luckily, you don?t have to work out the equations yourself: you can purchase astronomy software or use the U.S. Naval Observatory or SunEarth Tools to look up the sun?s altitude and azimuth.

Input a place name or geographic coordinates and a date into the forms on either Web site. A table will come up with the altitude and azimuth of the sun throughout the day. To correct for the atmospheric refraction and apparent size of the sun (an optical illusion that happens when the sun approaches the horizon), look at an altitude slightly below zero to find the azimuth of the sun at sunrise or sunset. If the sunrise or sunset azimuth matches your street azimuth, you?ve got a henge!

A reverse-lookup option for sunrise/sunset azimuth does not seem to be available, so you can?t ask the table what day the sun will have a particular azimuth. You?ll have to play around a bit to find the date of your henge. If you are looking for a sunset henge, check your location on June 20th if the grid is tilted clockwise and December 21st if the grid is tilted counterclockwise; these are the solstice dates and will let you know whether your street has a henge at all. (If your street axis is tilted farther from due east or west than the azimuth on the solstice, you?re out of luck.)

When you have determined whether there is a theoretical henge, you can adjust the date to pinpoint it precisely. Try moving the date a month away from the solstice and take note of how the sunrise or sunset azimuth changes. You?ll be able to adjust the date from there to find your henge. It takes a bit of practice, but you don?t need to be entirely precise. As with Manhattanhenge, there may be a few dates when the sun lines up with the street grid at different points of sunrise or sunset, and they may all be fun to watch.

To find a spectacular henge near you, look for an area with clear weather, imposing buildings or natural features, clear visibility to the horizon and straight roads. Chicago, with its skyscraper-filled downtown and near-perfect east?west grid, might yield beautiful sunrise henges over Lake Michigan on the equinoxes. Some Toronto residents have figured out that the 16-degree counterclockwise offset of their grid yields Torontohenge sunsets in February and October. And it should be possible for anyone reckless enough to walk to the middle of the busy Champs?Elys?es in the seventh or eighth arrondissement of Paris at sunset on August 12 to see the setting sun shining through the Arc de Triomphe!

Sunset looking down 34th Street, by Neil DeGrasse Tyson, 2001

If you?re in New York City and want to see Manhattanhenge, your best bet is to go as far east in Manhattan as possible along a wide east-west street. The Empire State Building on 34th and the Chrysler Building on 42nd create particularly striking views. You can also attend a program at the Hayden Planetarium on Wednesday night to learn more about Manhattanhenge.

For the rest of you, we want to know where you?re trying to find a henge. Is your street on a grid? Do you have a view of mesas in the Southwest, the Gateway Arch in Saint Louis or another man-made or natural feature that might frame the sun in a particularly beautiful way? If you figure out a location and date or get a great shot, tell us in the comments or post it on Twitter and tag it with #HereAHenge so we can see what you found.

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Source: http://rss.sciam.com/click.phdo?i=f82af8f47b6b35ccb63b7e6c353c4b91

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