Tuesday October 5, 2010

1. TuesdayOctober 5, 2010 (Moon Phases, Tides)

2. The Launch Pad Tuesday, 10/5/10 • Most Moon craters are produced by • volcanic activity. • impacts from meteoroids. • sinkholes caused by unstable lunar soil. • ancient lakebeds that are now dry. Identify the indicated features on the picture. C A B

3. Announcements • ??

5. Moon Phases

6. Moon Phases

7. Sunlight is shown coming in from the right. The earth, of course, is at the center of the diagram. The moon is shown at 8 key stages during its revolution around the earth. The dotted line from the earth to the moon represents your line of sight when looking at the moon. To help you visualize how the moon would appear at that point in the cycle, you can look at the larger moon image. The moon phase name is shown alongside the image.

8. One important thing to notice is that exactly one half of the moon is always illuminated by the sun. Of course that is perfectly logical, but you need to visualize it in order to understand the phases. At certain times we see both the sunlit portion and the shadowed portion -- and that creates the various moon phase shapes we are all familiar with.

9. Also note that the shadowed part of the moon is invisible to the naked eye; in the diagram above, it is only shown for clarification purposes. So the basic explanation is that the lunar phases are created by changing angles (relative positions) of the earth, the moon and the sun, as the moon orbits the earth.

10. Moon Phases

11. Tides Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth.

12. Tides Most places in the ocean usually experience two high tides and two low tides each day (semidiurnal tide), but some locations experience only one high and one low tide each day (diurnal tide). The times and amplitude of the tides at the coast are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean and by the shape of the coastline and near-shore bathymetry.

13. Tides

14. Tides The difference in height between high and low waters over about a half day varies in a two-week cycle. Around new moon and full moon when the Sun, Moon and Earth form a line, the tidal force due to the Sun reinforces that due to the Moon. The tide's range is then at its maximum: this is called the spring tide. It is not named after the season but, like that word, derives from an earlier meaning of "jump, burst forth, rise" as in a natural spring. When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the Earth, and the solar gravitational force partially cancels the Moon's. At these points in the lunar cycle, the tide's range is at its minimum: this is called the neap tide. Spring tides result in high waters that are higher than average, low waters that are lower than average, slack water time that is shorter than average and stronger tidal currents than average. Neaps result in less extreme tidal conditions. There is about a seven-day interval between springs and neaps.

15. Tides The changing distance separating the Moon and Earth also affects tide heights. When the Moon is at perigee, the range increases, and when it is at apogee, the range shrinks. Every 7½  cycles from full moon to new to full, perigee coincides with either a new or full moon causing perigean spring tides with the largest tidal range. If a storm happens to be moving onshore at this time, the consequences (property damage, etc.) can be especially severe.

16. Lunar Eclipses

17. Lunar Eclipses

19. What will happen to our Moon in the future? The Moon is gradually receding from the Earth, at a rate of about 4 cm per year. This is caused by a transfer of Earth's rotational momentum to the Moon's orbital momentum as tidal friction slows the Earth's rotation. That increasing distance means a longer orbital period, or month, as well.

20. What will happen to our Moon in the future? To picture what is happening, imagine yourself riding a bicycle on a track built around a Merry-go-Round. You are riding in the same direction that it is turning. If you have a lasso and rope one of the horses, you would gain speed and the Merry-Go-Round would lose some. In this analogy, you and your bike represent the Moon, the Merry-Go-Round is the rotating Earth, and your lasso is gravity. In orbital mechanics, a gain in speed results in a higher orbit.

21. What will happen to our Moon in the future? The slowing rotation of the Earth results in a longer day as well as a longer month. Once the length of a day equals the length of a month, the tidal friction mechanism would cease. (ie. Once your speed on the track matches the speed of the horses, you can't gain any more speed with your lasso trick.) That's been projected to happen once the day and month both equal about 47 (current) days, billions of years in the future. If the Earth and Moon still exist, the distance will have increased to about 135% of its current value.