Starship Asterisk*

The APOD of September 22 2008 is a very nice composite image. Clicking on that APOD however brings up a fuller view version which I think is even more effective and perhaps should have been used instead of the cropped version.

[quote="ozprof"]Just a complaint about the text under the APOD for today.

It says that "[i]Today is an equinox, a date when day and night are equal.[/i]".

This is NOT correct. [/quote]
An equineox: a quadruped where horse and ox are equal

[img]http://www.maijapeeples-bright.com/Art/Ceramics/EQUINE%20OX%202A%20WS.jpg[/img]

Am I the only person who has a problem with the following wording?

"The bottom Sun band was taken during the winter solstice in 2007 December, when the Sun [b][i]could not rise very high in the sky nor stay above the horizon very long. This lack of Sun caused winter.[/b][/i]"

Sure, during winter we have less sun time and a lower sun angle and less sun impingement total, thus the cold season. But the sun 'could not' rise high in the sky? This is like saying the earth 'could not' shift on its axis to cause a shorter or longer winter or summer. Ditto for the summer explanation.

Sorry to pick a nit, but this bugged me.

[quote="ozprof"]Hi all,

Just a complaint about the text under the APOD for today.

It says that "[i]Today is an equinox, a date when day and night are equal.[/i]".

This is NOT correct. If the Earth did not have an atmosphere, it would be correct.[/quote]
It would be wrong even in the absence of an atmosphere, because we define day by the beginning of sunrise, and the night by the end of sunset. The situation is further confounded by the fact that the equinox is a specific time, not a date, and it can fall any time during the day we call the equinox. Like you, I use the equinoxes as an example to my students about how refraction and our definitions contradict the "equal day and night" claim.

Nevertheless, I have no major problem with the assertion. On the whole, if people understand the reasoning behind this, they actually understand what the equinox means physically, which is more important than understanding the technical reasons that the day is actually a little longer than the night when we are near the equinox. I start by stating that the day and night are equal, and only move on to describe the error in this statement if my audience is receptive.

[url]http://apod.nasa.gov/apod/ap080922.html[/url]

An [url=http://en.wikipedia.org/wiki/Equinox]equinox[/url] is the moment in time ([i]not a whole day[/i]) when the centre of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 22 each year.

Then there's twilight. It can last all night in the polar regions, a long time as the sun's path cuts the horizon at a low angle in temperate regions, and a very short time as the sun's path cuts the horizon at a more vertical angle near the equator.

I hear that in Ecuador when the sun sets it gets dark real fast. Reminds me of Bullwinkle narrating: "Night fell." [i](thud)[/i]

[quote="ozprof"]Hi all,

Just a complaint about the text under the APOD for today.

It says that "[i]Today is an equinox, a date when day and night are equal.[/i]".

This is NOT correct. If the Earth did not have an atmosphere, it would be correct. As it is, atmospheric refraction causes the Sun to rise earlier and set later than otherwise. This makes the day longer than the night on the day of the equinox. To get the length of the day and night being equal, occurs either a few days earlier or a few days later than the equinox, depending on which equinox is occurring.

Sorry, but this has been a pet peeve of mine for many years and something that I make sure my students understand.

Cheers,

Ozprof[/quote]

You also need to consider the terrain, if the East and West elevations are not equal the horizon of a perfect sphere (if you are not living in the middle of the ocean or in Iowa) the day is shorter proportional the elevational differences to the spherical horizon.

Hi all,

Just a complaint about the text under the APOD for today.

It says that "[i]Today is an equinox, a date when day and night are equal.[/i]".

This is NOT correct. If the Earth did not have an atmosphere, it would be correct. As it is, atmospheric refraction causes the Sun to rise earlier and set later than otherwise. This makes the day longer than the night on the day of the equinox. To get the length of the day and night being equal, occurs either a few days earlier or a few days later than the equinox, depending on which equinox is occurring.

Sorry, but this has been a pet peeve of mine for many years and something that I make sure my students understand.

Cheers,

Ozprof

Not to knock the viewing from Turkey, but I think it's more the imager, Tunc Tezel. He produces a lot of really good images, and he just happens to live there. You can click on his name under today's APOD to find out a bit more on him.

Happy reading! 8)




*edit for spelling :oops:

Today has another great photo from Turkey, so many APOD's are coming from there - is that one of the best places in the world for astronomy and sky watching other than atop a mountain in the middle of the pacific?