# ManhattanhengeJuly 20, 2011 8:33 AM   Subscribe

How do I calculate when the sun will rise directly over a particular landmark?

There's a prominent east-west axis in my hometown that terminates at an important building. It seems logical that the sun will rise directly behind the building at some point.

How do I calculate about what day this happens?

Thanks!
posted by jefficator to Sports, Hobbies, & Recreation (8 answers total) 4 users marked this as a favorite

The Photographer's Ephemeris will do this, I believe.
posted by jquinby at 8:38 AM on July 20, 2011 [1 favorite]

If the axis is due east-west, the sun will rise directly behind the builiding on the equinoxes.

If it's not quite true east-west, you need to find out how far off (and which way) it is from the true direction, and convert that to an azimuth (due east = 90°; south of east is greater than 90°, and north of east is less than 90°). Once you have the azimuth, you can use this sunrise and sunset calculator, with the option to show the azimuth. Find the dates on which the azimuth of sunrise equals the azimuth of the building.

I did the calculation for downtown Toronto in this AskMe a few years back.
posted by DevilsAdvocate at 8:46 AM on July 20, 2011

To dovetail off of DevilsAdvocate, you can measure the bearing of a street with Google Earth's ruler tool. I just used it (and the sunrise/sunset calculator) to discover that the sun will set on the west end of our town's main street on 9/12.
posted by jquinby at 9:15 AM on July 20, 2011

DevilsAdvocate is absolutely correct, if you are standing due east or west of the building.

From your point of view, the Sun will rise to the east of *you* on the equinox, but if you're, say, northwest of the building, the sun won't appear to rise behind the building. However, the building's shadow will still be cast along the road, if the road stretches due west from the building. (Reverse east and west for this paragraph if you're thinking of sunset instead.)
posted by BrashTech at 9:17 AM on July 20, 2011

Hadn't heard the term Manhattanhenge before. It was posted to the blue ages ago as Gotham Equinox.
posted by etc. at 9:21 AM on July 20, 2011

Also, my answer assumes a clear view beyond the building to the "true" horizonâ€”imagine that beyond the building is nothing but ocean, or a large lake, or a large flat plain. If there's other buildings, or mountains, or even trees (if they're close enough) behind it, that changes things.

Imagine that you're in an area with a mountain range to the east. The day on which your first view of the sun is due east may be off from the equinoxes by days or even weeks, since you won't see the sun until it's above the mountaintops, possibly several degrees above the theoretical horizon. It looks like you could probably do it with The Photographer's Ephemeris as suggested by jquinby, possibly with a bit of trial and error, to find the date and time at which the sun appears at the azimuth of the building, and the altitude necessary to appear to you (blocked by nothing other than the building).
posted by DevilsAdvocate at 9:54 AM on July 20, 2011

It looks like you could probably do it with The Photographer's Ephemeris as suggested by jquinby, possibly with a bit of trial and error, to find the date and time at which the sun appears at the azimuth of the building, and the altitude necessary to appear to you (blocked by nothing other than the building).

You can do just that, apparently - there's a video that shows this precise use-case based on altitude of the viewer. It's on the About page, towards the bottom.
posted by jquinby at 9:59 AM on July 20, 2011 [1 favorite]

As DevilsAdvocate said, be careful of altitude. It has a huge effect on sunrise/set timing - far more sensitive than latitude.
posted by IAmBroom at 10:42 AM on July 20, 2011

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