Bakersfield College

November 3, 2012

Bakersfield Night Sky – November 3, 2012
By Nick Strobel

As I write this column, Sandy has made landfall in New Jersey with storm surges made more extreme by the fact that it is a Full Moon today/tonight. My wife suggested that I write about why high tides are more extreme at Full Moon (and New Moon too). Tides are caused by different strengths of gravity acting on a body. The force of gravity gets weaker with distance. For the Earth, the major tide producer is the Moon. The side of the Earth facing the Moon is about about 4000 miles closer to the Moon than the center of the Earth is, and the Moon's gravity pulls on the near side of the Earth more strongly than on the Earth's center. This produces a tidal bulge on the side of the Earth facing the Moon (the "near side" of the Earth). The Earth rock is not perfectly rigid; the side facing the Moon responds by rising toward the Moon by a few centimeters on the near side. (When the Moon formed about 4.5 billion years ago, it was only 40,000 miles away and the tides were extreme: the Earth's rock surface rose and fell up to 200 feet in a day!) The more fluid seawater responds by flowing into a bulge on the side of the Earth facing the Moon. That bulge is the high tide.

At the same time the Moon exerts an attractive force on the Earth's center that is stronger than that exerted on the side away from the Moon. The Moon pulls the Earth away from the oceans on the far side, which flow into a bulge on the far side, producing a second high tide on the far side. These tidal bulges are always along the Earth-Moon line and the Earth rotates beneath the tidal bulge. When the part of the Earth where you are located sweeps under the bulges, you experience a high tide; when it passes under one of the depressions, you experience a low tide. The tidal cycle at a given location also depends on the latitude of the site, the shape of the shoreline, winds, etc. The Sun's gravity also produces tides that are about half as strong as the Moon's and produces its own pair of tidal bulges. They combine with the lunar tides. At new and full moon, the Sun and Moon produce tidal bulges that add together to produce extreme tides. These are called spring tides (the waters really spring up!).

Storm surges are primarily the result of high winds pushing the water on the ocean's surface. The lower air pressure at the center of a hurricane or other storm system also has an effect, though it is smaller. The combined effects of high winds plus low air pressure makes the storm surge. Now add to Sandy's storm surge the tides due to the Moon and Sun acting together and you have the makings for a particularly abnormally large storm surge. Not a good time to be near the east coast.

Tides happen wherever there is gravity, so that's everywhere in the universe. Tides are responsible for the heating of the interior of some of Jupiter's large moons, especially Io, with more active volcanoes erupting than any other body in the solar system, and Europa, where the tides warm the water enough below the icy surface to create a deep ocean of liquid water possibly tens of miles deep. Tides are also responsible for the tearing apart of stars that get too close to a galaxy's central supermassive black hole as appears to have happened in one distant galaxy under observation by two X-ray space telescopes, the JAXA/NASA Suzaku and the ESA XMM-Newton and possibly even in our own galaxy as seen by the NuSTAR space telescope.

Our evening sky this evening and for the next week or so doesn't have any bright objects visible until Jupiter become visible a little before 8:30 PM (7:30 PM when daylight savings ends on Sunday). Jupiter is still between the horns of Taurus and continues moving backward (retrograde) or westward among the stars—see the second star chart below. It will reach the Hyades in Taurus' head by the third week of January. In November we will see Jupiter get even brighter than it is now. That's because we are catching up to and passing by it during this month. Earlier in the evening you may be able to spot Mars among the stars of Ophiuchus (the "13th zodiac constellation") low in the southwest but it will probably require binoculars to spot it among all of the twilight glow—see the third star chart below. By the end of next week Mars will have moved into Sagittarius. Jupiter will dominate the sky until 10 PM tonight when the Waning Gibbous Moon rises next to the legs of the Gemini twin, Pollux. On November 6th, the Moon will be in third (or last) quarter phase.

For this November, the time to observe is in the early morning an hour or two before sunrise, so that is what is shown in the first star chart below. Venus will become visible by 4:15 AM (standard time—remember to set your clocks back one hour before you go to bed this evening!). Although Venus is getting a little dimmer than it was the previous two months, it is still brighter than Jupiter that will be high in the western sky by then. Venus will close in on the bright star Spica in Virgo, getting closest by November 17th. Tomorrow morning, Saturn rises at about 6 AM and it will be hard to see without binoculars in the dawn twilight. A thin Waning Crescent Moon will pass by Venus to the right on November 11th and on the 12th, an even thinner crescent Moon will pass by Saturn. Later in November we'll see Venus and Saturn have a close conjunction under darker skies because Saturn will rise up earlier.

Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
--
Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com

Kern Community College District