Saturday, October 11, 2008

Estimating Lunar Date in Singapore

I derive this simplified formula myself based on what I know (only works in Singapore - other areas have to use the other formulas)

Lunar date (estimated) = Time (1st 2 digits in 24 hr time) - 4 - Position of Moon from East
(+24 for negative value)

Lunar month (usually) = Gregorian Month - 1
(hence January is 12th month)

take the first 2 digits of 24 hr time when you see the moon. Eg. If it is 11am, it is 11. If it is 7pm, it is 19
Number 4: It is constant (a correcting factor) (most other tropical countries will be 5)
Position of Moon from East: Direction East will be 0. Direction West will be 14. Directly above head will be 7. The rest is divided accordingly.

Please allow +-1 for estimated date of lunar date.


Example1: You see the moon at around 45 degree above ground in direction of East at 10pm in the month of May

Lunar date (estimated) = 22 - 4 - 4 = 16
Lunar month (usually) = 5 -1 = 4

So it is 16th of the 4th lunar month

Example 2: You see moon at around 10-20 degree above head to the direction of West at 9am in the morning in the month of december

Lunar date (estimated) = 9 - 4 - 8 = -3 = 21 (after adding 24)
Lunar month (usually) = 12 - 1 = 11

So it is 24th of the 11th lunar month

Please note that this is only an estimation as the position of moon depends a number of factors and it is not a constant. That is why some lunar holiday may change date as it gets closer.

Some affecting factors (there may be more)
1) Tilt of Moon's orbit with respect to the Earth
2) Location of Earth with respect to the Sun (Gravitation pull from the Sun is not constant as Earth's orbit is not completely circular)

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