Expert: Jayendra Upadhye - 7/23/2005

Question
Hey! This is Rahul, I have some questions for you, hopefully you will be a good source for an answer instead of these random sites with no direct answer. When is the Moon visible in the daytime sky? How does its position change in respect to the Sun from during the period from new to full moon to the period after the full moon? What would be the best time to observe the Moon in the daytime sky? Thanks for all of your help.

Answer
Hi,
Truly speaking I have never given a thouht to this.
1 - When is the Moon visible in the daytime sky?
ans.
Imagine a plane bisecting the earth, and which is at right angles to the line joining the centers of the earth and the sun.
The day-night line is the cross section of the earth that belongs to that plane. (The earth's axis is tilted by 23.5 deg to this plane).

The moon circles the earth once every 28 days (approx) and goes around its orbit in the same anticlockwise direction when viewed "down" from above its orbital plane.

The plane containing the circular day-night line of the earth also bisects the moons orbit into two semi-circles, one on the nightside of the earth, the other on the day side of the earth.

The moon is visible during daylight hours for most of the month.
sometimes setting in the west, a few hours after sunrise.
sometimes rising early in the eveining, before sunset.
Only on fullmoon day, it "just rises" when the sun sets and "just sets" when the sun rises, leaving one WHOLE DAY with no moon.
This too is only a technicality, as the grey area of the moon being visible atleast for a few moments is VERY real.

You can verify what i am saying by actually noting moonrise-moonset times and sunrise-sunset times, and checking out the overlaps.
Any Panch-ang will help as these contain pre-calculated accurate full tables in them! Thanks to Hindu interest in astronomy!

In sanskrit terminology, (SHUKLA PAKSHA WHEN MOON BECOMES BRIGHTER BY THE DAY FOR 15 DAYS AFTER AMAVASYA, KRISHNA PAKSHA MOON BECOMES WEAKER FOR 15 DAYS AFTER POURNIMA..ACTUALLY THE PERIOD IS SOMEWHERE BETN 14 AND 15 DAYS)..middle of "Krishna-Paksha".
By the time the Krishna-paksha culminates into tghe Amavasya of "new-moon", the moon is directly overhead. That is why solar eclipses occur on amavasya day.

Conversely, In the middle of shukla-paksha, the moon crosses over from the dayside half of its orbit into the nightside half of its orbit. This culminates into the "pournima" of full moon. That is why we have lunar eclipses on pournima days (full moon).

2 - How does its position change in respect to the Sun from during the period from new to full moon to the period after the full moon?
Ans
new-moon moon between sun and earth, slightly above or below the earth's ecliptic.
mid shulkla paksha "half waxing moon",  moon crossing over to earth's nightside part of its orbit.
full-moon, moon directly on opposite side of the sun with earth between. Slightly up or below the ecliptic.
mid krishna-paksha or "half waning moon", moon crossing over to earth's dayside part of its orbit from the nightside part of it's orbit.

3 - What would be the best time to observe the Moon in the daytime sky?
Ans
Obviously one does not have to be a rocket scientist to know that if one wants to observe the moon during daytime, following must hold true. (moon in opposite position of sun in sky)
  a) mid krishna-paksha (approx 7 days after pournima), "half-moon" is directly overhead at sunrise, one may watch it till it sets at middday!
  b) Till amavasya, no point as moon is in "sunward" half of the sky ie, eastwards of overhead!
  c) Amavasya to mid shulkla paksha..same as b, but moon is westwards and towards the sun. westwards of overhead.
  d) mid shukla paksha (approx 7 days after amavasya), "half-moon" directly overhead in the evening. One may watch it as it rises at midday and climbs overhead at sunset.
  e) - For mid shukla paksha till pournima, one may watch it as it rises later and later in the afternoon, till the sun sets.
(to meet the criteria "watching during day".
  f) - In first half of krishna paksha (just after pournima), the moon may be observed after sunrise,  on the west as it gains altitude daily.

In short, Best time to view the moon (when you see more of it) is "middle of shukla paksha to middle of krishna paksha" or when it is in the nightside half of its orbit. At these times, one can see it "full" in the east on full moonday sunset. And progressively waxing and waning to "half moons" states, in gereat comfort as the sun will be in opposition.

Hope this satisfies.
Jayen