Weather forecasts have it feeling like summer for a while longer, but summer officially ends here in Utah and the rest of the northern hemisphere on Friday, Sept. 22 at 10:03 p.m. MDT (12:03 a.m. on Saturday, Sept. 23 Eastern Time).
According to NASA Solar System Ambassador to Utah Patrick Wiggins, "At that moment the sun will glide southward across the celestial equator, an event known as the September equinox."
The celestial equator is an imaginary line in space directly above Earth's equator.
On the day of the equinox the periods of daylight and dark are nearly equal and the sun rises due east, and sets due west, which Wiggins notes, "Can be a problem for drivers in Utah where so many roads run due east and west."
While we in the northern hemisphere think of this equinox as the start of the shorter, cooler days of fall, those in the southern hemisphere, where the seasons are reversed, see it as the start of the longer, warmer days of spring.
The next similar event, known as the March equinox, will occur when the sun moves northward across the celestial equator next March, marking the start of northern hemispheric spring.
Similarly, there are two times a year when the sun is furthest from the sky's equator. One is at the start of summer, when it's furthest north, and the other is at the start of winter, when it's furthest south. These events are known as the June and December solstices.
For further astronomical information visit Wiggins' NASA Solar System Ambassador website at http://utahastro.info.