Portage County sky events for September
Sept. 1 – Sunrise 6:21 a.m., sunset 7:35 p.m.
Sept. 2 – First-quarter moon.
Sept. 8 – Full moon (Harvest Moon); sets at sunrise.
Sept. 15 – Last-quarter moon.
Sept. 22 – Autumnal equinox; sun above equator.
Sept. 24 – New moon; passes just below the sun.
Sept. 30 – Sunrise 6:55 a.m., sunset 6:41 p.m.
We are reminded of the rhythm of the seasons. For me, summer is marked by the feeling of new life as fresh air blows through the open windows of my home. It ends with fresh cucumbers and school buses making their return through the neighborhoods. The weather becomes perfect for outdoor activities.
Every September, a dramatic change occurs all around us. The weather changes; it gets darker sooner. Some insects and plants die. Others ready themselves for the onslaught of winter. People wear cheese on their heads. It gets weird.
We have the Earth’s tilt to thank for these changes. For the last six months, Earth’s northern hemisphere has been tilted toward the sun as it spins on its axis. It’s like a flower that turns to the sun. We have summer as sunshine lasts longer and the sun’s energy reaches us more directly.
On the evening of Friday, Sept. 22, the sun will be directly overhead on the equator. Because of this, most locations on the Earth will receive roughly equal day and night. This is the origin of the term equinox, or “equal night.” The sun also sets nearly due west and rises nearly due east the morning of the Sunday, Sept. 23.
It isn’t exactly true, however, that we have equal time for day and night. Even on the equator, an effect called refraction (from the atmosphere) bends the light of the sun and therefore lengthens the time the sun is visible for an earlier sunrise and later sunset.
This means that the sun is visible for seven minutes longer than it would if we had no atmosphere. The size of the sun and your latitude will also factor into how much daylight there is.
What does the autumnal equinox mean for the Southern hemisphere? This day marks the beginning of spring as the days get longer and the sun gets higher in the sky. A bias is evident here and a push is being made by some to replace the term with simply the “September Equinox.”
If you are like me and have to pretend that you enjoy the dark winter to make it through sometimes, there is a bright side.
Just like a ball thrown into the air spends more time higher up than it does lower down (where it is moving faster), the Earth spends more time between the vernal (or spring) equinox and the autumnal equinox than it does vice versa (or versa vice as my Spanish teacher always said).
Count the days between Sept. 22 and March 20 and you will see that the seasons are not symmetrically divided.
Earth is moving fastest around the sun in January when it is at its closest point, known as perihelion, and thus spends less time in this part of its orbit. The difference of approximately five days may not mean much to you, but I’m taking it.
The full moon on Monday, Sept. 8, is known as the Harvest Moon. It is the full moon closest to the equinox and can occur in September or October. The full moon following the Harvest Moon is called the Hunter’s Moon.
Their meanings are self-explanatory, as the light from the moon allows for the harvesting of crops throughout the night, or the hunting of animals for winter preparation.
The return of Aurora Borealis
The sun also has a rhythm of its own and we are intimately connected to it. You probably know about sunspots or have heard of solar flares. Think of them as weather on the sun.
When the sun is active, as it is moderately now, there can be dozens of sunspots. When it is very active, there can be over 100. In late 2008, I took my physics class outside to witness them using a telescope and couldn’t find any!
The cycle is approximately 11 years in length and scientists have observed nearly 30 of them since observing began in the early 1600s. We are currently experiencing the cycle known as SIDC 24.
Because of the angle the Earth makes relative to the sun in September and March, a slight effect allows for better aurora, commonly known in the northern hemisphere as the northern lights. As cycle 24 wanes over the next few years, the chances of seeing aurora are better now than they will be for some time.
Other than the slight effect during Earth’s equinoxes, the sight of auroras is dependent mostly on the sun and darkness. Usually, auroras are associated with winter, but that is because days are especially long during summer in far northern latitudes. Winter is darker and makes aurora easier to see. Cold nights are also not necessary. Nights that are clear with low humidity allow for both colder nights and better viewing of aurora. The two are not directly related.
Unfortunately, a number of factors keep forecasting time limited to the time it takes the solar wind (mostly protons) to reach the Earth after an observed solar flare. This prediction time can be from several hours up to a limit of three days. Because the sun spins at a rate of once every 25 to 30 days (the sun spins faster at its equator), an aurora event can repeat itself when the sun spins back around and shoots another blast of particles and energy toward Earth.
For a wonderful program about the Earth’s aurora, visit the Allen F. Blocher Planetarium at the University of Wisconsin-Stevens Point during its public shows. These shows are held on Sundays, with “Aurora!” starting Sept. 14 and running through Oct. 12. Check the web site for times or schedule changes at www.uwsp.edu/physastr/plan_obs.
Editor’s note: Dan Hagstrom teaches physics at Stevens Point Area Senior High School. GNATS is the acronym for “go now and teach someone.”