By Andy Veh, for the Redoubt Reporter
Graphic courtesy of Andy Veh
Days are getting longer and nights are getting shorter. Thus, this will be my last column before fall.
The winter constellations Orion, Gemini, Taurus, Canis Major and Auriga with all their bright stars are now visible in the west, setting during the late evening. Leo with its bright star Regulus is speeding across the sky, which is why I perceive Leo as the harbinger of spring. When it appears in the east, winter’s end will soon be here. When it reaches the western horizon, flowers are in full bloom and deciduous plants will have regained their leaves. In addition, the summer triangle comprised of the bright stars Vega, Deneb and Altair reappears in the northeast.
As it has been for most of this winter, it’s going to be another great month for planets. During March we saw superbright Venus in the west right above Jupiter. Although Jupiter is the second-brightest object in the sky ahead of even the brightest stars, the giant planet just about fades in comparison. Venus keeps moving farther left of Jupiter night after night — a really good example showing that planets move (the word “planet” means “wanderer”).
What really has surprised me is how high Venus appears, even at midnight. My students learn that the inner planets — Mercury and Venus — can only be visible for some time after sunset or before sunrise, and since they are close to the sun, they have to set or rise just a bit later or earlier. But Alaska is different in some respects. As Venus moves through the summer Zodiac constellation of Taurus right now, it does appear high above the horizon (just as Taurus does), setting five hours after the sun. That’s akin to the almost midnight sun that we see on the Kenai during summer.
On April 21 and 22 the crescent moon is close to Jupiter, and on April 24 it’s close to Venus.
Red Mars is left next to Leo’s Regulus, producing a pretty pair high in the south. They are joined by the waxing gibbous moon April 3 and by the first quarter moon April 30.
Low in the southeast are Virgo’s Spica and Saturn, with the full moon very close April 6 and the waxing gibbous moon nearby May 3 and 4. During one month neither Mars nor Saturn move much. Spica and Regulus are fixed stars (relative to all other stars), thus, the moon ends up in the same places. Therefore, these time intervals reflect the moon’s orbit of 27 days around Earth, while completing its phases takes two days longer.
The other planets are too close to the sun this month, because they are either close to upper conjunction (on the other side of the sun) such as Uranus, Neptune and Pluto, or too close to the Alaska horizon when the sun rises or sets, such as Mercury and Venus.
In past columns I talked about lunar phases and about eclipses, so here’s another moon topic: Why do we have tides on Earth? Specifically, why is there a tide on the side of Earth that is facing away from the moon?
Tides are caused by gravitational force. The moon exerts a larger gravitational force on the Earth’s side facing the moon than the center of the Earth because that side is 4,000 miles closer. So, water is in a sense pulled away from the Earth (rock is affected, too, but water is much more flexible and can flow). Likewise, the moon exerts a larger gravitational force on the Earth’s center than on the Earth’s side facing away because the center is 4,000 miles closer. So the Earth is in a sense pulled away from the water on the other side. Thus, the reason is the same for both tidal bulges — gravitational force.
Here’s a demonstration: Set up an Earth globe and a moonlike object a distance away. Hold two small blue sponges (any two objects would work) onto the Earth — one on the side facing the moon, the other on the side facing away from the moon. Pretend that the moon in your model exerts a gravitational force by moving the facing sponge 6 inches toward the moon and moving the Earth globe 5 inches toward the moon (it’s less because the Earth’s center is farther away, so there’s less gravitational force). Move the other sponge 4 inches toward the moon (it’s less because the Earth’s far side is farther away, so there’s less gravitational force). Now you have two tides, each 1 inch in height, explained with a quite elegant model.
By the way, the water needed to produce the high tides sloshes from the left and right sides of Earth where there would be low tides at the same time.
Since we live near the coast, we are familiar with the tides — that we have high tides about every 12.5 hours as the Earth goes through half a rotation. Or, in other words, from high to low it’s a little over six hours, and the same time back to high. The two daily tidal cycles repeat every 25 hours, because the moon advances in its orbit, making tides about one hour later every day.
Because the tidal force is the difference of gravitational forces between two localities, the computation and results are different (the gravitational force itself decreases with the square of the distance, but the tidal force decreases with the cube of the distance). So the moon’s influence on the tides is more important than the sun’s, despite the moon’s much smaller mass.
Last year in April I answered this question from Jason Daniels’ Kalifornsky Beach Elementary School class: “Is the moon the only thing that affects the tides? Or is it just the main thing?”
The moon is the main body that produces tides on Earth, being responsible for two-thirds of a tide. The sun produces one-third of a tide. During new moons and full moons, when the sun, Earth and moon are aligned and both bodies pull in the same direction, those fractions add and we get extra-large tidal ranges (good clamming during the extra-low tide). During first and last quarter moons, when the sun, Earth and moon are at right angles and both bodies pull in different directions, those fractions subtract and we get extra-small tidal ranges.
While there are misconceptions about the reasons for the seasons and why the moon exhibits phases (see my past columns for the correct explanations), the greatest misconception — judging by the flood of literature and websites purporting it — seems to be about the tide on the other side. No, it has nothing to do with centrifugal force, and any book mentioning that got it wrong.
Here is an ingenious and simple model that explains tides: http://www.youtube.com/watch?v=CTQ6ciHENgI (in “Explain-it: How do the tides work?” the BBC’s Adam Hart-Davis uses a cookie, a piece of string and a pickle). For a really in-depth explanation, check out http://www.lhup.edu/~dsimanek/scenario/tides.htm.
By the way, Daniels is spending this year in Australia, seeing the same planets and some of the same stars, yet a completely different view of his southern sky. For him, the sun still rises in the east and sets in the west, but he probably had to get used to seeing the sun in the north in the middle of the day.
Andy Veh is a professor of astronomy and physics at Kenai Peninsula College’s Kenai River Campus.
