The summer heat wave has ended for most of North America. There might be a little nip in the night air, and the leaves are starting to turn to their fall colors. Autumn is coming to the northern hemisphere. The autumnal equinox, which marks the first day of fall in the northern hemisphere and the first day of spring in the southern hemisphere, will fall on September 23, 2011.
The solstices and equinoxes are not actually days, but rather they are instants of time. The autumnal and vernal equinoxes are the instants when the Sun appears directly over Earth's equator. The summer and winter solstices are the instants when the Sun appears overhead at the furthest north or south location on Earth.
For 2011 the moment of the autumnal equinox is September 23 at 09:05 UTC (coordinated universal time), according to the 2011 Astronomical Almanac. Coordinated universal time is also called Greenwich Mean Time (GMT), the standard time in the Greenwich, England time zone. In the United States, the autumnal equinox will be at 5:05 am EDT, 4:05 am CDT, 3:05 am MDT, and 2:05 am PDT. For locations a few time zones further west, the 2011 autumnal equinox occurs on the late night of September 22.
The seasons are reversed in the northern and southern hemispheres because the seasons are caused by the tilt of Earth's axis. Contrary to what many people think, the changing distance between Earth and the Sun has no effect on causing the seasons.After a hot summer, enjoy the cool fall weather if you live in the northern hemisphere. If you live in the southern hemisphere, enjoy the warm spring weather.