AMHERST, Mass. – The public is invited to witness sunrise and sunset associated with the autumnal equinox among the standing stones of the UMass Amherst Sunwheel on Saturday, Sept. 22 at 6:30 a.m. and 6 p.m. These Sunwheel events mark the astronomical change of seasons when days and nights are nearly equal in length in the Northern Hemisphere.
UMass Amherst astronomer Stephen Schneider will discuss the astronomical cause of the sun’s changing position during the hour-long gatherings. He will also explain the seasonal positions of the Earth, sun and moon, and answer questions about astronomy, such as why sunrises and sunsets are at different times for the fall and spring equinoxes and why days and nights are not precisely equal on the equinox.
The exact minute of the autumnal equinox this year is 9:54 p.m. Eastern Daylight Time on Sept. 22. This marks the moment that the sun crosses the celestial equator from north to south as seen from Earth, ushering in the beginning of fall in the Northern Hemisphere. On the day of the equinox, an observer located on the equator will see the sun pass directly overhead at local noon, and it marks the beginning of six months of daylight at the South Pole and six months of nighttime at the North Pole.
On any day other than the equinox, either the Northern or Southern Hemisphere is tilted toward the sun. For observers, except those at the north and south poles, the sun on the equinox – for equi, “equal” and nox, “night” – rises due east and sets due west and stays up for 12 hours and down for 12 hours. From the Sunwheel in Amherst, observers standing at the center of the standing stones see the sun rise and set over stones placed to mark the equinoxes.
If the skies are clear, telescopes will be set up to permit observations of the sun and other objects during the evening session. The moon will be nearly full, rising just shortly before the sun sets. About half an hour after sunset, it may be possible to view the crescent Venus, which is passing Earth in its orbit, moving from the evening to the morning sky. Jupiter, Saturn and Mars are also visible in the evening sky.
The UMass Amherst Sunwheel is located south of McGuirk Alumni Stadium, just off Rocky Hill Road about one-quarter mile west of University Drive. Visitors to the Sunwheel should be prepared for wet footing, and mosquito repellent is highly recommended. Heavy rain cancels the events. Donations are welcomed and will be used to help with the cost of additional site work at the Sunwheel and future events.