When does summer end and fall start ?

Answer 1

The seasons in the northern hemisphere are the opposite of the seasons in the southern hemisphere.

There is a table of all seasons for each year
from 1900 to 2099 on my astronomy web site here:
http://neoprogrammics.com/astronomy/

In the left menu select:
[Table of Times of The Seasons]

The time that 'kevinngunn' gave (9:51 UT) for the beginning of autumn in the northern hemisphere is correct, except that it is for next year, not 2006. This year, autumn starts at 04:03 UT on 23 September.


A note about times:

Actually UT and GMT are not technically equivalent.

Proper UT is used for scientific purposes and daylight saving or summer time is not applied. That is, GMT is adjusted for summer or daylight saving time, while proper UT is not.

Right now (GMT - UT) = 1 hour

The general public often treats UT and GMT as identical in all cases, but this is not strictly correct. Only when daylight saving or summer time is NOT in effect, then is UT = GMT

The public uses GMT, scientists use UT for technical purposes for consistency and to avoid the confusions of having to account for summer or daylight saving time.
 

Answer 2

The start of fall begins with the autumnal equinox. This year it occurs on September 23 at 9:51 UT (to convert to EST subtract four hours). The autumnal equinox is the time when the sun crosses the celestial equator heading south (for those of us in the Northern Hemisphere). For folks in the Southern Hemisphere our autumnal equinox is their vernal equinox as they are moving into spring.

Answer 3

the autumnal equinox is usuall around Sept. 20 or 21. That's when fall starts.

Answer 4

On the Autumnal Equinox -- 9/23 this year.

Answer 5

Autumnal Equinox. Sun is at a right angle to equator of the earth.

Answer 6

All seasons are related to the tilt of the Earth and the orbital path around the Sun.

Answer 7

September 21st, officially anyway.
But I think summer already ended :(