It's time for the seasons to change

It's that time again. Time for a seasonal change.

Although it has felt like summer for a few weeks now, it won't be official until Sunday, June 21, at 10:38 am MDT when the Sun reaches the furthest point north in its travels for the year -- on the Tropic of Capricorn -- 23 degrees north of the equator.

At that time the Sun will stand high overhead at noon there (unless they have Daylight Saving Time there). If we were still using the old Celtic calendar they would call it Mid-Summer's Day.

That is the Summer Solstice, solstice coming from a Greek word meaning "Sun stands still."

Now the Sun does not actually stand still on that day, it is just that it seems to rise very close to the same place each day for several days.

The cause of our seasons is that pesky 23.5 degree tilt of Earth's axis. Because of that tilt we in the Northern Hemisphere are receiving more direct sunlight, thus warmer temperatures.

Conversely, the good folks in the Southern Hemisphere are being tilted away from direct sunlight and they are entering the winter season.

There is something else that pesky tilt is responsible for--something called "Precession of the Equinox."

Earth's axis not only tilts, it wobbles. Sort of like a toy top when it is slowing down. The direction the axis points changes with this wobble. That means the current North Star -- Polaris -- has not always been the North Star.

In 3,000 BC, the North Star was Thuban, a star in Draco the Dragon, in 12,000 BC it was Vega, in Lyra, the Harp, and will be Vega again in 14,000 AD.

As can be guessed from the name, the location of the equinox also changes. This movement was discovered by the ancient Greek astronomer Hipparchus in the second century BC.

Since then the location of the equinox has moved westward against the background stars, about the distance of one zodiacal constellation. Back in that day the summer equinox was in Cancer, now it is between Gemini and Taurus.

Something else you may want to consider. All you folks who thought you were Leo's (like me), you may now have to consider that you are really Cancers.

SKY WATCH: New moon, July 16. Remember, a new moon means no moon is visible. That is also a good time to start our moon watch for the month.

Go outside each evening after local sunset and look for the moon close to where the Sun went down. The first night you may not see anything, but by Thursday you will have no problem locating the thin crescent of the moon. Then is a good time to look for Earth Shine, the reflection of sunlight off of earth back onto the surface of the moon. It is best observed with binoculars.

It gives a rather ghostly look to the unlit portion of the lunar surface. It will get better for the next couple of nights. Keep a record of your observations. Where was the moon, what time was it observed, what did it look like. You could even draw a picture of what you see. It doesn't need to be a masterpiece, just a representation of what you saw. Keep it up for the entire lunar cycle--this will mean getting up early in the morning sometimes. Just remember, good science is worth it. You could also be surprised to discover that the moon is visible as much in the daytime as after dark. Try it yourself and see.

NEXT WEEK: The great Jupiter/Venus meeting is coming and you don't want to miss it, and more astronomical blathering.