Equinox day: in Singapore and the tropics

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starfinder
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Equinox day: in Singapore and the tropics

Post by starfinder »

I was reminded this morning that today (20 March 2013) is the day of the vernal equinox, when I browsed the excellent Astro Bob website and read this informative article:
http://astrobob.areavoices.com/2013/03/ ... e-station/

As can be seen from this screenshot from the Daylight World Map App (Android) set for today at astronomical local noon in Singapore (1.11pm), the Sun was then very nearly overhead Singapore here in the tropics:
Image

I took the opportunity to take this overhead photo of the Sun at precisely 1.11pm in the Raffles Place area today, and indeed it showed the Sun almost exactly overhead:
Image

See, there were no slanted shadows!
Image

To be precise, since Singapore is not exactly on the equator but at 1 deg 17 min North, the Sun at local noon today was not at altitude 90 degrees above the horizon, but transited the meridien at 88.5 deg instead; the Sun was then at celestial declination -0 deg 10 min.

I should think that only on the equator itself does the Sun ever reach 90 deg above the horizon on the days of the equinoxes (20 March and 20/21 Sept); someone correct me if I'm wrong.

Also, it's only those places located between the Tropic of Cancer and the Tropic of Capricon which may experience the Sun crossing exactly 90 deg overhead (almost) at local astronomical noon, i.e. on the 2 days of the year that the celestial declination of the Sun is the same as the geographical latitude of that place. Thus for Bangkok (13 deg 45 min North), that happens on 27 Apr and 16 Aug each year when the Sun's declination is at around +13 deg 45 min: go ahead, try it out on your planetarium program 'cause I've confirmed it on mine!

And since Singapore's latitude is 1 deg 17 min North, those 2 corresponding days are 24 March and 19 Sep every year: on those days the Sun would be at precisely 90 deg above the horizon at local astronomical noon (assuming that the Sun is then precisely at declination +1 deg 17 min). So, this Sun 24 March 2013, when the Sun transits at 1:10:19pm (declination +1 deg 24 min), it would be at altitude +89 deg 45 min above the horizon in Singapore, i.e. just 15 arc mins from being absolutely overhead.

There! Isn't wonderful to live in the tropics...

Any questions or comments on the above? Have I left anything out?
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Gary
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Re: Equinox day: in Singapore and the tropics

Post by Gary »

Nice illustration using the shadow of the car! Are there any sundials in Singapore that can show such nice effects? Botanic Gardens?
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