Nakshatra - position of moon in the sky
We have a tradition of observing birthdays
based on the nakshatra. So, many of us
observe an "English" birthday and
a "star (nakshatra)" birthday.
What is Nakshatra ?
What does it mean when one says Shri Rama
was born in Punarvasu nakshatra ?
What does it mean to say,
today is Uttara Ashaadhaa nakshatra ?
Literally, nakshatra means star.
However, in the context of nakshatra of a day,
the literal meaning makes no sense. All the stars
are always up there. So, what is special about
Punarvasu on the day Shri Rama was born ?
The moon was around Punarvasu nakshatra
on that day. The figure below shows the position
of the moon on such a night. Facing east, you can
see it in the sky on the night of Punarvasu nakshatra.
In the figure, the group of stars left of the moon
is considered Punarvasu.
The next such night is on 5 Feb 2012.
(the lines are guides, meant to easily identify the stars)
The nakshatra defines the position of the moon in the sky.
What has a star to do with position in the sky ?
From our point of view, the stars in the sky are fixed.
Each part of the sky can be identified with the prominent
stars visible in that region.
Suppose you watch the moon every night at around the same time.
On day 1, it would be around a group of stars.
On day 2, it would have shifted further east from the previous day.
This shift continues until on day 28, it is in the same position as day 1.
Another way to look at it - Divide the sky into 27 parts.
Name each part based on brightest star(s) visible in that part.
The moon sequentially stays one day on each of those parts.
Each days nakshatra is the name of the part in which the moon stays.
When one says today is Rohini nakshatra, it means
the moon can be seen in that part of the sky
where one finds Rohini nakshatra.
(Today is Amaavaasyaa / new moon day.
Cannot see the moon.
Try watching the sky three days later)
So, nakshatra defines the position of the moon in the sky.
There are 27 nakshatras.