Ask an Alim

Alaykum Salaam.

The  Jurists  believe  that when Allah states, “And they ask you about
the  crescents, tell them that they are time references for people and
for  the  hajj  [pilgrimage]…”  He is addressing people in general and
this  serves  to  guide  them  to take the crescent as a measure, or a
reference  point,  for the start of their religious or Islamic months.
As  such,  it  is  unwise  to  depend upon the opinions of astronomers
without  Allah  (swt)  or  His  Prophet  (saw) guiding people to do so
[either  through  the  sayings,  actions  or  authorization  of  the
infallible,  therefore,  Allah],  for  it is an ancient and well-known
science to many, pre-Islamically.

Undoubtedly,  the  jurists pay special attention to follow every piece
of  information,  and any scientific advancement and use it as long as
it  does  not  conflict  with  the  religious texts. For example, they
accept  the  opinion of the astronomer when he states that there is no
possibility of a sighting. In addition, the jurists accept the opinion
of  the  astronomer  when  he  states  that  the  crescent sighting is
dependent  upon the atmospheric conditions, such as the clarity of the
skies,  that  there  are no barriers such as dust, or smoke that is if
the  astronomers  opinion  gives  one contentment (which is a state in
which  one  does  not  have  complete certainty, however, he does have
enough  information that leads him to feel comfortable or content with
the  information).  However,  the problem here is with the astronomers
and  not  with  the  jurists  because  the  astronomers  have  great
differences amongst themselves in assessing the time most suitable for
the possibility of a sighting, rather they emphasize the difficulty in
attaining contentment themselves. Refer to the following link:
http://aa.usno.navy.mil/faq/docs/crescent.php

Jaffer Dharamsi