FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

All Things Being Equinox

On Wednesday evening, autumn begins. But what does that really mean? Let's take a little quiz to see how much you know about the season already.

T/F Autumn starts on Wednesday because it is the day exactly midway between the first day of summer and the first day of winter.

False. Autumn, or fall, begins on September 22 because Earth strikes a unique pose relative to the sun on that day. You may recall that the "first day" of summer is when we in the northern hemisphere are tilted most towards the sun in our orbit. The "first day" of winter is when we are tilted most away from our star. From summer to winter we are going from a "tilted most towards" position to one "tilted most away." Almost between those two extremes - but not quite - is when we are no longer tilted with respect to the sun at all; we are standing straight up. That is how we define the first day of autumn.

T/F The first day of fall is also called the solstice.

False again. The first days of summer and winter are called solstices. On Wednesday we observe the "equinox." Because of this "nontilt" day, the sun hits Earth from pole to pole. All of us on Earth who are not at a pole will have the sun up for 12 hours and down for 12 hours. We are all equal on that day. The term "equinox" itself means "equal night," probably from the fact that the night and the day are equal now.

T/F After the first day of fall, the days get shorter and the nights longer.

Trick question, sorry. The daytimes have been getting shorter since the first day of summer actually. That is when we were tilted most toward the sun. After that we in the northern hemisphere have been slowly tilting away. This has the effect of shortening our daytimes, the hours that the sun is up. After the first days of fall, the time that the sun is below horizon beats out the time it is above the horizon. You will definitely see a more pronounced effect in the day/night difference in the next months, but the daytimes have been gradually shortening since June.

T/F Many plants can sense this pronounced shortening of daytime and get themselves ready for a cold winter.

True! As the days shorten significantly, the biochemistry of many trees and shrubs responds in such ways as to shut down food production in leaves. (Leaves from many species of trees are too delicate to survive wintertime.) This changes their colors and eventually closes down the leaves all together causing them to fall off the tree - which is most likely the origin of our North American word for autumn, "fall."

Until next time, clear skies - and a wonderful beginning to a new season!

The Big Guy is Showing Off

Jupiter is out and is providing us with some free sky activities, things you and your family can do to make watching the sky a little more interesting than just the occasional glance upwards.

Some of us have already noticed that intensely bright planet rising in the east in the early evening. It is Jupiter, not to be confused with that other brilliant object setting in the west after sunset, which would be Venus.

Jupiter is preparing to own the fall/winter months as Big Planet in the Skies. Of course, Jupiter is always a nice target for even a small telescope with its huge, striped disk almost popping into a scope's field of view. Just a pair of binoculars can pick out the four Galilean satellites hugging the big guy.

But we can use Jupiter to demonstrate some of the other traits of our night skies, using nothing more than the naked eye.

For example, you may notice that Jupiter is rising about the same time that the Sun is setting. This means the two are on opposite sides of our planet. We call this time opposition. Good news for scope owners: Opposition means we are passing as close to Jupiter as we will get, so you can get a fine view of the planet during the month.

If you're a trooper you can peer up periodically through the night as Jupiter slowly rises in the sky, eventually passes overhead, then sets in the west by sunrise. This is something we take for granted but it is, of course, the Earth rotating on its axis giving us the appearance that the entire dome of the sky is moving over us. Jupiter is a great sky marker for this kind of observation.

We can also use Jupiter to draw out a unique, imaginary line in the heavens called the ecliptic. The Moon and Venus will help us here.

Starting on Saturday, the 11th, go out at about 8 PM and see in the western skies a low Venus near a crescent Moon. On the opposite side of the skies, in the east, you should see Jupiter.

Now notice each night, about 8 PM, how the Moon has moved slowly but steadily towards Jupiter, finally reaching the gas giant on the 22nd. This imaginary line between Venus and Jupiter which the Moon has followed is called the ecliptic, and it is on or about this line, which goes around Earth, that you will find all the planets.

Moreover, doing this little exercise will help you see the daily movement of the Moon as it orbits around our planet.

There's a load more things you can enjoy by observing the skies but these little activities involving Jupiter can give you an idea of how early astronomers began to put together big pictures of how this amazing universe works.
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me