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Old 05-10-2019, 07:44 PM
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spacezebra (Petra)
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A challenge for the astute astronomer

Greetings all...I have a challenge!

I have a friend who is trying to determine the exact time that Napoleon B crowned himself emperor.

What we know is the date: 2 December 1804
We know the location is Notre Dame (ND):
He arrived the ND at 11.45am
The ceremony was over by 3.00pm

Using the painting by Jacques - Louis David (titled Napoleon's Coronation).. we have a a sun shadow in the painting...but that all we have.

The challenge is... work out the approximate time of the actual coronation.

Cheers Petra d.
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Old 05-10-2019, 08:16 PM
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mental4astro (Alexander)
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Petra,

I'll be Devil's advocate here and suggest that the painting is not a reliable source for a time record. The work is designed and composed entirely for purposes of grandure, pomp, ceremony and effect. Guests alone would not have been gathered around as depicted in the painting - it's not a photographic record. The painting would have taken months to complete, and as such would have been composed & designed entirely for dramatic effect. This includes the angle of incidence of the Sunlight. It could also be very likely that the "sunlight" in the painting might actually be impossible to have occured to illuminate the altar as shown. It is a scene that is entirely composed for effect. Contrived.

Alex.
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Old 05-10-2019, 08:34 PM
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spacezebra (Petra)
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Thanks Alex

Even though the painting is not a reliable source (which I agree), I posed the challenge to see if, based on the painting an approximate time of coronation could be calculated.

So if just based on the painting.....

Cheers Petra
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Old 05-10-2019, 09:26 PM
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mental4astro (Alexander)
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As a geometric exercise, certainly possible. Will require knowing the orientation of ND to the Sun (easy enough), and then the pillar in the background that has the beam of sunlight across it, and then work backwards to work out the timing/elevation of the Sun so it would cast that illumination. Not so straight forward as the angle of illumination on the pillar is not the actual angle of elevation of the Sun.

Interesting problem you've proposed, Petra

Alex.
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Old 06-10-2019, 12:45 PM
ab1963 (Andrew)
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It has to be later in the afternoon as the sun must be setting, Sunset in France at the beginning of December is just short of 5pm so looking at the shadow would say 2.30pm......
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Old 06-10-2019, 07:24 PM
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Quote:
Originally Posted by ab1963 View Post
It has to be later in the afternoon as the sun must be setting, Sunset in France at the beginning of December is just short of 5pm so looking at the shadow would say 2.30pm......
Many thanks for the response.

Cheers Petra
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Old 06-10-2019, 09:16 PM
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Hi Petra & All,

Looking at the balcony/awning in the upper left corner of the painting and assuming it is level (ie the same as the horizon) and measuring the angle of the shadow it casts with a protractor, the angle of the shadow is 36 degrees -- indicating the altitude of the Sun is about 36 degrees.

However, Paris is at approximately 48 degrees N and given the Sun is in Ophiuchus on 2 December at about 22 degrees S declination it means the maximum altitude it attains on 4 December is about 20 degrees.

You cannot determine the time from the painting because the Sun is at an impossible angle for 4 December.

Best,

L.

Last edited by ngcles; 07-10-2019 at 05:10 PM.
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Old 07-10-2019, 09:49 AM
N1 (Mirko)
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Quote:
Originally Posted by ngcles View Post
Hi Petra & All,
Looking at the balcony in the upper left corner of the painting and assuming it is level (ie the same as the horizon) and measuring the angle of the shadow it casts with a protractor, the angle of the shadow is 36 degrees -- indicating the altitude of the Sun is about 36 degrees.

That would only be the case if the incident angle of sunlight was parallel to the wall holding the "balcony" (I think it's some kind of awning). Other angles, and the shadow angle on the wall would not be the same as the sun's altitude (at 90° the shadow axis would appear vertical on the wall regardless of the sun's altitude). Looking at the cathedral's orientation, layout and interior, I would agree that no direct sunlight would have been reaching the area most brightly lit in the painting. The awning might have been illuminated by the afternoon sun through the upper windows on the southern side, but the dull look of the light casting that shadow compared to the scene in front of the altar lets me concur with Alex: any clues to actual lighting have been lost to glorification.
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