MIT Infinite Corridor Astronomy

Prepared by Ken Olum

As viewed from a stationary point on the earth, the path of the sun through the sky is roughly a circle which moves north and south as the seasons go by. In mid-November and in late January every year, the circular path crosses the axis of the infinite corridor. When this happens the setting sun can be seen from the far end of the corridor. (The same phenomenon cannot be seen at sunrise because the other end of the infinite corridor is blocked by building 18.)

Predictions

The predictions shown below give the times at which the center of the sun will cross the azimuth of the infinite corridor axis, and the observed altitude of the center of the sun above a level reference plane at that time, in minutes of arc. The sun is approximately 30 minutes of arc (half a degree) in diameter, and so extends about 15' in each direction. Because there is a hill in Boston that blocks a level view along the corridor, only parts of the sun that are somewhat above the level plane are visible.

Predictions for 2000-2001

Date            Time     Altitude
 11/11/2000    4:19:23    43'
 11/12/2000    4:20:38    23'
 11/13/2000    4:21:53    5'

Date           Time     Altitude
 1/27/2001    4:50:32    2'
 1/28/2001    4:49:37    21'
 1/29/2001    4:48:39    40'

Predictions for other years.

Warning

The fact that the sun is at the horizon does not mean that it is safe to look at for more than a moment. Do not injure your eyes by staring at the sun. Use appropriate filters or glance only for a moment as you would at the sun up in the sky.

What you'll see

As the sun becomes better and better aligned with the corridor the amount of floor that is illuminated goes up and up. Since the floor is very reflective this means that the reflected sun can be seen far down the corridor as the event approaches. The orange light reflected onto the ceiling is often striking. This phenomenon is visible for a week or two around the actual sunset days.

At some point, some portion of the sun's disk will be visible from the far end of the corridor. Which part of the disk will be seen first depends on whether the sun is crossing above or below the corridor. If the altitude in the table above is high, the sun will cross the upper right edge of the corridor as seen from the lower left. If the altitude is low, the sun will cross from the center left of the corridor and set below the hill.

The disk of the sun is larger than the area of sky visible from the end of the corridor. Therefore if your timing is right and you get a good vantage point you will see nothing of the sky except for the interior of the sun's disk.

Observing this event depends strongly on atmospheric conditions. I think the best is a very red sunset. This decreases the total light intensity and makes it possible to see the corridor and the sun at the same time. If the sky is brilliantly transparent right down to the horizon then the setting sun will be too bright to look at. When you use a filter to save your eyes (see below) you won't be able to see much of the corridor.

I have found that the infinite corridor produces the "moon illusion", in which the setting sun looks gigantic in the sky, even though it is no larger at the horizon than it is at the zenith.

The period of time during which the sun can be seen from the corridor end is quite short -- no more than two minutes, and only as long as that under ideal conditions.

Some suggestions:

View from the building 8 end.: Don't go up to the Mass. Ave. end of the corridor. You won't see anything special there, and you will prevent others from seeing.
Use the third floor.: The 3rd floor has the best view because it has an uninterrupted sweep of corridor, and because it is the highest usable floor. You can't see anything from the fourth floor because it doesn't go through.
Let others see.: Not very many people can get a good view of this phenomenon at once. Once you have seen, get out of the way and let others look
Stand in the stairwell.: The place that has the largest unobstructed angle of view of the sky is the stairwell just below the third floor. Get your eye right by the edge of corridor floor. Great view? Now give someone else a chance.
Try building 18: This event can also be seen from building 18, looking out through the building 18 hallway windows and in through the window at the end of the infinite corridor. Opposite the doorway to 18-421P is about the right place. You will need to crouch down near the floor. It might also be possible to see it from the third floor of building 18, but then you might need a ladder.
Don't hurt your eyes: See the warning above.

Data and Computations

These predictions depend on knowing the azimuth of the infinite corridor to an accuracy of several arc-minutes. The best figure we have found is the accurately surveyed azimuth of MIT's property line along Memorial Drive, which is 245.47391075 degrees true. According to Michael K. Owu '86, the infinite corridor should have been constructed parallel to this line. However, it is possible that they aren't really exactly parallel, which would cause an error in the above tables. Other possible causes of error include computer program bugs and general incompetence on my part.

You can help improve the computations by taking sightings during the event. What we need to know is the azimuth of the corridor. The azimuth of the sun in the sky at any given time can be easily computed. So if you observe that the left edge of the sun is visible by sighting along the corridor wall and make a precise note of the time then we can calculate the azimuth. Note that this doesn't help with the top edge, but only the left or right edge. Also note that we need a fixed line in order to know what it is whose azimuth we've computed. It won't help to say "I saw the left edge disappear behind the door" unless you know from what point you observed this.

MIThenge

The arrangement of the infinite corridor so as to capture the setting sun at a particular moment is sometimes called MIThenge by analogy with Stonehenge.

Up to MIT

kdo@alum.mit.edu