Shadow of a doubt

October 31, 2005 12:28 AM

AtronomyFilter involving the Sun, Shadows and a *SPOILER* from "National Treasure".

Okay ... one of the key scenes in the movie "National Treasure" reveals an object found when the shadow of a steeple points at a specific brick in a wall at a specific time.

Would a solar shadow ALWAYS cast such a consistent shadow all year 'round, decade after decade? Isn't there even a slight deviation? I suppose sundials wouldn't work if it did deviate ... still, it seems counterintuitive.

posted by RavinDave to Science & Nature (14 answers total)

The only thing I can imagine that would have an effect is the precession of the equinoxes. According to this page (answering a similar question) it doesn't.

So it looks as if National Treasure is safe, for this part.
posted by sbutler at 1:04 AM on October 31, 2005

I'm sorry, but it looks like I didn't read the question all the way.

You will not get the same shadow every day of the year. The sun rises over different points of the horizon during different parts of the year. I vividly remember this, because in HS I had a 15 minute drive through corn fields in the morning. During the winter the sun rose in line with the road, but during the summer it rose slightly to one side. Made for some difficult driving, especially with frost on the window :).

So, if they had included a specific day or range of days, then I don't see what's wrong with the point. But you won't get the same shadow, at the same time, for an arbitrary day of the year.
posted by sbutler at 1:14 AM on October 31, 2005

That's the boner you're concerned about? When they show a multi-story deep basement under Lower Manhattan (and thus sea level) that is astonishingly dry? ;-)

(At least Nic Cage wasn't outrunning the sun like Brendan Fraser in The Mummy 2 ...)

Certainly, you would have to account for precession and other eccentricities of Earth's orbit, though I doubt the shadow position would change much over 200-odd years -- the Earth's axis precesses on a schedule of 26,000 years. Maybe one degree's difference. There are other wobbles to account for -- the Earth bobs up and down due to the orbit of the Moon, for instance -- but they're fairly constant and on smaller scales.

The main problem with the movie, however, is that there was no such thing as coordinated time when the treasure would have been buried. That only came into being with the railroads, which needed consistent schedules. Before that, every community relied on "local time" determined, roughly, by the Sun. The NT guys would have had to figure out what the local time was first. Using a modern watch is nonsense.
posted by dhartung at 1:15 AM on October 31, 2005

Response by poster: Nice link.

I can almost wrap my mind around the apex of a steeple shadow consistently crossing a specific brick in a wall -- by always at 2:21 pm? (allowing for artificial things like time-zone or daylight-savings changings, etc.)
posted by RavinDave at 1:16 AM on October 31, 2005

Response by poster: For the record -- I'm not picking nits with the film ... merely using it as a concrete example for the astronomy question.
posted by RavinDave at 1:20 AM on October 31, 2005

Consider that some peoples have determined the time of year based on variations in shadow positions. There are petroglyphs in the American SW that are positioned to match the shadows of nearby rock formations on the summer or winter solstice.
posted by brundlefly at 1:24 AM on October 31, 2005

You might want to check out some stuff on stonehenge. The stonehenge aotearoa has a obelisk that casts a shadow which follows the path of the figure 8 at different times of the year. I can't remember whether it was a certain time or certain angle of the sun that determined when the shadow would hit the line. Seems like a copy of the petroglyphs brundlefly mentions.
posted by scodger at 1:53 AM on October 31, 2005

I suppose sundials wouldn't work if it did deviate ...

Sundials are set at a particular angle. The building is vertical.
posted by AmbroseChapel at 2:16 AM on October 31, 2005

Correct me f I'm wrong, but wouldn't sunlight passing through a hole in almost any shadow-casting object make a figure eight as the year progressed?
posted by Kirth Gerson at 2:49 AM on October 31, 2005

As Kirth Gerson says, sunlight forms an analema on horizontal and presumably on verticle surfaces in the course of a year. I believe that an object would cast a shadow in the same place each year w/in the parameters mentioned by others above.
posted by Hobgoblin at 5:13 AM on October 31, 2005

Another recent movie that incorporated the analemma was Tom Hanks' through a small opening in his cave in Castaway. And although it may change and be inaccurate over the course of thousands of years, it's fairly accurate over a few hundred, at least.
posted by UnclePlayground at 6:36 AM on October 31, 2005

If you look at the sun (not directly of course) over the course of a year you will see that the arc it appears to transcribe over the course of the year will get lower into winter then extend again post-winter solstice until the summer solstice when the sun will start and end the day at the widest possible points on the arc and appear higher in the sky. After this it the size of the arc will reduce once again. If a fixed point on the Earth casts a shadow the change in the position of the light source means the place where the shadow falls will inevitably car over the course of the year. Now obviously the Earth keeps going round the sun every year and the pattern of rise and fall of the arc repeats - there is thus potential for the shadow to fall in the same place at the same time two years in a row. If observers as in National Treasure wish to take advantage of this they would have to know at what time and date the brick was chosen in order to find it again. Over time it may be that 'variation' in planetary movement might mean the shadow moves away from its original position on a particular time/date.
One interesting example of someone possibly taking advantage of this yearly occurrence is the possibly apocryphal story that Isambard Kingdom Brunel, a well known 19th Century engineer built a railway tunnel through Box Hill such that the sun shone through it on his birthday. The link discusses how likely it is that he did and also how the position of shadows can vary over time.
posted by biffa at 6:36 AM on October 31, 2005

UnclePlayground writes "Another recent movie that incorporated the analemma was Tom Hanks' through a small opening in his cave in Castaway."

Quick question about this: did Hanks' character have a watch? You need to be able to measure clock time to observe an analemma, right?
posted by mr_roboto at 10:56 AM on October 31, 2005

Mr_roboto, I inscribed one on my floor by marking the spot of light cast by a washer hung in my south-facing window every Sunday at noon. One can take a pretty good guess at when noon is.
posted by Hobgoblin at 2:14 PM on October 31, 2005