Expert: Philip Stahl - 11/11/2008

Question
Hi,

When we observe the sky after sunset, we see only 3 or 4 stars. As night arrives, we see more stars in the sky. Why is such a delay in observing all stars in the sky? Also in somedays, even at midnight, we see less number of stars while at other nights, we see lots of stars? Please clarify the reason.

Regards

Answer
Hello,

Relatively fewer stars are visible earlier in the evening because of astronomical twilight which can last up to an hour after sunset. Past twilight many more stars are visible because you have the condition of true darkness (this is assuming, of course, you don't live near a large metropolitan area full of lights. One of the primary banes of modern astronomical observing - whether amateur or professional- is "light pollution")

The most plausible reasons for seeing different stars even at midnight at the same location may be:

(i) increased pollution or dust in the atmosphere at those times,

(ii) more light pollution, or maybe a full Moon (or quarter Moon) which diminishes the ability to see fainter stars (mag. +2 or higher)

(iii) high cirrus clouds which either blot out the fainter stars or reduce visibility of brither ones at those times