Astronomy Blogs Timeline

This is cool (he says, catching up with something a few days old):

Stuart, over at Astronomy Blog, has use SIMILE's timeline code to produce a timeline of astronomy blogs

I'm the Gnomon

I spent last weekend in Lancaster. While I was there I took a walk into Williamson Park so I could go and take some photographs of a pretty neat sundial they've got there.

When someone mentions "sundial" most people probably think of the more "traditional" kind of sundial that has a fixed gnomon. The Lancaster Sundial is a little different in that it's an analemmatic sundial and you play the part of the gnomon.

In the middle of the dial is an analemma. To tell the time, and to take the equation of time into account, you pick a spot to stand on depending on the date and then use your shadow to figure out the time (adding an hour if needed depending on daylight saving time).

You can see more images of the sundial in my album of photographs of Lancaster.

Ignoring the Sun

Ever since I wrote about the UK floodlighting petition back in November I've been keeping an eye on the e-petitions site (its RSS feed makes this nice and easy). Some petitions are obvious, some are amusing, some are kooky and some are just outright strange.

I saw one this morning that probably falls under amusing, or perhaps just misinformed:

Why bother changing the clocks? Greenwich Mean Time is valid all year round. Midday indicates that the sun is highest in the sky; changing to BST over the summer suggests that we should ignore what the sun does when we plan our lives.

Nice enough idea I guess but there's a problem with the reasoning. There's a reason why Greenwich Mean Time is called Greenwich Mean Time — I'll borrow a paragraph from Wikipedia to explain why:

Noon Greenwich Mean Time is not necessarily the moment when the Sun crosses the Greenwich meridian (and reaches its highest point in the sky in Greenwich) because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT (this discrepancy is known as the equation of time). The fictitious mean sun is the annual average of this nonuniform motion of the true Sun, necessitating the inclusion of mean in Greenwich Mean Time.

In other words, even when we're working with GMT, we're still "ignoring what the Sun does" (well, not really, but I'm trying to use the line of thought of the author of the petition).

And now for the gratuitous use of the word analemma, which gives me the perfect excuse to link to Anthony Ayiomamitis' amazing images.