Saturday, January 31, 2009

Local Apparent Time (LAT)

To give you some idea of how big the Bandung Sundial is - whose bottom view of its observation deck I posted yesterday - today I'm posting these photos.

This first photo shows the view from the observation deck. As the you can see, the shadow of the gnomon (the sundial's "clock hand") shows that the time is almost 12:00 o'clock - although the term "o'clock" of course would not really be appropriate here as the time is not shown by the clock, but by the sundial.

The time shown by a sundial is called LAT or Local Apparent Time, that is the time as shown by the apparent motion of the sun. Because the earth is not a stationary object in space and because it has an elliptical orbit around the sun, the length of each day is not the same everyday and in different parts of the earth. Therefore, the near noon time shown by this sundial (which is located at Latitude -6.843 and Longitude 107.485) and at the time I took this photo (18 January 2009) will not exactly coincide with nearly 12:00 o'clock noon at other locations that belong to the same time zone as we know it and at a different time of the year.

To get an accurate conversion from LAT to the local clock time, we first need to find the Local Mean Time (LMT), which can be calculated with the following formula:

LMT = LAT - ∆

where ∆ is the value of the equation of time (there is a table for this).

The LMT we get will then need to be fine-tuned against the location's actual longitude.

If you're still confused with all this, then the most important thing is - I think - the basic principles of how it works.

By the way, this second photograph is the bridge that surrounds the sundial. The strips you see here are the minute markers.


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Anonymous said...

Wow..that is huge Eki..My husband can always tell the time by the position of the sun and he gets very, very close to the right time..

Mahmud Yussop said...

Very interesting information. I like the idea that it can also show the minutes on the ground. I guess this is a must place to see in Bandung. BTW you're tagged by Inul @ htpp:// Have a lovely day.

Babooshka said...

That is enormous. Really does put the dial n context.

the donG said...

beautiful how you merged the two photos. it is quite difficult to do that.

Lisa B. said...

That is so cool! Thanks for the lesson!!