Starship Asterisk*

by **Chris Peterson** » Tue Dec 23, 2008 6:04 pm

JohnD wrote:They certainly weren't taken at local noon.
There's a map of the Acropolis at http://www.acropolis-greek-parthenon.com/ (bottom of page).
It shows that the Erectheion is on the northern lip of the outcrop, and that the Porch faces North by West a few degrees.
So the analemma picture is taken by a camera facing almost directly South East.
Will such a picture taken at, say 1030 local time, show this pattern?JOhn

It doesn't matter when you take the picture. As long as you use the same local time each day, you'll get the analemma. If you take the image too close to sunrise or sunset, part of it might be below the horizon for some of the year, however.

I don't know the exact details for this particular image, but Anthony has explained in the past how he does his analemma shots. He usually shoots the analemma sequence without concern for the foreground (which is so underexposed as to be invisible) and then sets up the foreground shot. He gets the angles correct, so the analemma is properly located, but doesn't necessarily take the sequence and the foreground from exactly the same location.

by **JohnD** » Tue Dec 23, 2008 5:50 pm

They certainly weren't taken at local noon.
There's a map of the Acropolis at http://www.acropolis-greek-parthenon.com/ (bottom of page).
It shows that the Erectheion is on the northern lip of the outcrop, and that the Porch faces North by West a few degrees.
So the analemma picture is taken by a camera facing almost directly South East.
Will such a picture taken at, say 1030 local time, show this pattern?

JOhn

by **BMAONE23** » Tue Dec 23, 2008 5:00 am

It might become clearer if you think of it as an image of the sun taken at 8 am on every Monday morning for an entire year (that would be 52 images of the sun though the picture contains 46) then spliced together so that the building images line up in the background. As the sun tracks lower (to the northern hemisphere) in the sky during winter and higher in the sky in summer, it will appear at the top of the pattern at 8 am during summer and at the bottom at 8 am during winter.
Interesting though about the picture, the building is illuminated from our vantage point but the sun is depicted behind it so the side we see should be in shadow.

by **Chris Peterson** » Tue Dec 23, 2008 4:51 am

Frenchy wrote:I guess what I am trying to say is that I am trying to visualize the analemma in three dimensions in my head (to give the sun's motion depth that the photo doesn't provide). In essence, what shape is the sun's orbit as the Earth/Sun system moves through space?

This isn't a three-dimensional problem. The Sun isn't orbiting anything, or moving at all. The Earth is orbiting the Sun, and the Earth is the only thing moving. The figure-eight pattern consists of two components: the north-south component is simply the declination shift in the Sun's position over a year, caused by the axial tilt of the Earth (the Sun is highest at the summer solstice, and lowest at the winter solstice); the east-west component is caused by the difference between local mean time and solar time- because the clock and the Sun aren't perfectly synchronized.

There is a slight "depth" in the figure simply because the Earth's orbit is slightly eccentric, so the distance to the Sun isn't quite constant. But this is a small effect and has nothing to do with the existence of the analemma. The Earth could be in a perfectly circular orbit and you'd still get the figure-eight (but with equal sized lobes). In that case, the Sun would be at exactly the same distance in each exposure.

by **Frenchy** » Tue Dec 23, 2008 3:18 am

I guess what I am trying to say is that I am trying to visualize the analemma in three dimensions in my head (to give the sun's motion depth that the photo doesn't provide). In essence, what shape is the sun's orbit as the Earth/Sun system moves through space?

by **apodman** » Mon Dec 22, 2008 4:39 am

starnut wrote:The sun is on the other side of the temple, so the sides we are seeing in the photo should be in shade!

Unless the analemma sun-dots were shot in the afternoon and the temple was filled in by the light of the morning sun.

That is indeed the case, since the small (northerly) loop of the figure-8 is to the right and so the view is to the west.

That is also the way to get the best shot.

I might even be tempted to use a sun filter on my camera when shooting the sun-dots. If the short exposures alone don't already do so, surely the filter will cast the temple beyond shade into total darkness.

by **starnut** » Mon Dec 22, 2008 4:01 am

More picky comments. I have never visited Athens and so have no idea of the temple's orientation with respect to the sun, but methinks that the photographer could have picked the more appropriate sides of the temple when making the composite picture. The sun is on the other side of the temple, so the sides we are seeing in the photo should be in shade!

Gary

by **apodman** » Mon Dec 22, 2008 1:02 am

Case wrote:The sun-dot most close to horizon seems to be slightly outside of the analemma figure. Compositing error, perhaps?

If you draw a smooth best-fit figure-8 curve through the sun-dots, you will see several sun-dots almost as far off the curve as that one. This one may be farther off than others, but it also sticks out because of its prominent location. Errors can come from inaccurate timing, wind pressure on one side of the camera, imperfectly realigning the camera after it was disturbed, knocking the camera while covering or uncovering it, or whatever. I don't have a location myself where I could set up a camera outdoors and expect it not to be disturbed for a full year.

Compositing? The old way was with film and dedicating a camera for a year to repeatedly expose the same frame. I suppose compositing must take place with digital cameras, but at what stage I don't know. Somebody please tell me.

Frenchy wrote:What is causing the analemma or sun to change planes?

I don't understand this question. Can you please explain or rephrase?

by **Frenchy** » Mon Dec 22, 2008 12:23 am

What is causing the analemma or sun to change planes?

by **Case** » Sun Dec 21, 2008 11:11 pm

The sun-dot most close to horizon seems to be slightly outside of the analemma figure. Compositing error, perhaps? Because I find it more unlikely that the sun shifted somewhat on that day.

by **apodman** » Sun Dec 21, 2008 6:29 pm

thearborist wrote:The caption explains that changing the time or location would change the appearance of the analemma.

The size and shape of the figure-8 remain the same. Only the analemma's sky location and angle change with time and camera location.

thearborist wrote:What change will there be over the course of several years? Will the next year be identical? How long until the analemma repeats exactly?

Over the course of several years, if you leave the camera in the same place and take pictures at the same time, the sun will trace the same figure-8 over and over. The actual sun positions along the figure-8 where the sun can be photographed each day will vary from year to year, but will stack up in four sets according to our leap year schedule.

As the Earth's axial tilt changes over thousands of years, the shape of the figure-8 will change.

As the angle made between the major axis of Earth's elliptical orbit and the alignment of the equinoxes changes over thousands of years, the shape of the figure-8 will change.

As the Earth's orbit changes over millions or billions of years, the shape of the figure-8 will change.

by **thearborist** » Sun Dec 21, 2008 5:33 pm

The caption explains that changing the time or location would change the appearance of the annalemma. What change will there be over the course of several years? Will the next year be identical? How long until the annalemma repeats exactly?

by **bystander** » Sun Dec 21, 2008 3:48 pm

JohnD wrote:OK, picky, picky, but this is more strictly termed the "Caryatid Porch" (21st December 2008).

"Caryatids" were devotees of Artemis the Huntress, rather than Athena Polias - "City (of Athens) Protector)" - to whom, with Poseidon, the Erechtheum was dedicated but the term has become used for any supporting pillar carved in a female shape. To call the structure the "Porch of the Maidens" implies some significance to their inviolate status, as if the cult of Hestia had similar civic functions to the Roman Vesta cult, which it didn't. They may portray Athena Parthenos, but that implies her virgin birth, not her chastity.

In today's APOD wiki link to Erechtheum, the porch is actually called The porch of the Caryatids. However, the wiki link to Caryatid says, The Greek term karyatides literally means "maidens of Karyae", an ancient town of Peloponnese. So it would seem "Porch of the Maidens" is still appropriate.

by **neufer** » Sun Dec 21, 2008 3:36 pm

http://apod.nasa.gov/apod/ap081221.html

JohnD wrote:OK, picky, picky, but this is more strictly termed the "Caryatid Porch" (21st December 2008).

"Caryatids" were devotees of Artemis the Huntress, rather than Athena Polias - "City (of Athens) Protector)" - to whom, with Poseidon, the Erechtheum was dedicated but the term has become used for any supporting pillar carved in a female shape. To call the structure the "Porch of the Maidens" implies some significance to their inviolate status, as if the cult of Hestia had similar civic functions to the Roman Vesta cult, which it didn't.

They may portray Athena Parthenos, but that implies her virgin birth, not her chastity.

-------------------------------------------
Cary & Anna: the little Lemma sisters
...........................................
Analemma , n. [Gr. ανάλημμα, any supporting pedestal of a sundial, fr. to take up; + to take.]

A caryatid (Greek: Καρυάτις, plural: Καρυάτιδες) is a sculpted female figure serving as an architectural support taking the place of a column or a pillar supporting an entablature on her head. The Greek term karyatides literally means "maidens of Karyae", an ancient town of Peloponnese. Karyai had a famous temple dedicated to the goddess Artemis in her aspect of Artemis Karyatis: "As Karyatis she rejoiced in the dances of the nut-tree village of Karyai, those Karyatides, who in their ecstatic round-dance carried on their heads baskets of live reeds, as if they were dancing plants"

http://en.wikipedia.org/wiki/Caryatid
-----------------------------------
Arte Mis(ter) Neuendorffer

P.S., OK, picky, picky, but where is the Daylight Saving time glitch?
........................................
<<Explanation: If you took a picture of the Sun at the same time each day, would it remain in the same position? The answer is no, and the shape traced out by the Sun over the course of a year is called an analemma.>>

<<Greece is in the Eastern European Time Zone. Eastern European Standard Time (EET) is 2 hours ahead of Greenwich Mean Time (GMT+2). Like most states in Europe, Summer (Daylight-Saving) Time is observed in Greece, where the time is shifted forward by 1 hour; 3 hours ahead of Greenwich Mean Time (GMT+3). After the Summer months the time in Greece is shifted back by 1 hour to Eastern European Time (EET) or (GMT+2)

http://wwp.greenwichmeantime.com/time-z ... on/greece/

by **JohnD** » Sun Dec 21, 2008 12:28 pm

OK, picky, picky, but this is more strictly termed the "Caryatid Porch" (21st December 2008).

"Caryatids" were devotees of Artemis the Huntress, rather than Athena Polias - "City (of Athens) Protector)" - to whom, with Poseidon, the Erechtheum was dedicated but the term has become used for any supporting pillar carved in a female shape. To call the structure the "Porch of the Maidens" implies some significance to their inviolate status, as if the cult of Hestia had similar civic functions to the Roman Vesta cult, which it didn't.
They may portray Athena Parthenos, but that implies her virgin birth, not her chastity.

And nice analemma, but haven't a lot of APODs been so?
John

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Analemma Over the Porch of the "Maidens" (2008 Dec 21)

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Cary & Anna: the little Lemma sisters

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