The Moon. Nicola Loaring, SAAO

Transcription

1 The Moon Nicola Loaring, SAAO

2 Vital Statistics Mean distance from Earth Orbital Period Rotational Period Diameter 384,400 km days days 3476 km (0.272 x Earth) Mass kg ( x Earth) Density 3340 kg/m 3 Surface Gravity 0.17g

3 The Near and Far Side

4 Surface Features

5 The Moon s Rotation

6 Why do we only see one face of the Moon? Tidal locking: A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This synchronous rotation causes one hemisphere constantly to face the partner body. Usually, at any given time only the satellite is tidally locked around the larger body, but if the difference in mass between the two bodies and their physical separation is small, each may be tidally locked to the other, as is the case between Pluto and Charon.

7 Day and night on Earth

8 Moon Phases

9 The Southern View!

10 Moon Phases

11 Why do the phases repeat every 29.5 days?

12 Moon phases and time

13 The influence of the Moon - 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. Most places in the ocean usually experience two high tides and two low tides each day.

14 Tides The difference in height between high and low waters over about half a 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: spring tide. When the Moon is at first quarter or third quarter, the solar tidal force partially cancels the Moon's. The tide's range is at its minimum: neap tide.

15 Tidal Locking The Moon's orbital distance from Earth is gradually increasing is due to the influence of the tides. The Earth makes a complete rotation about its axis in only 24 hours while it takes 27.3 days for the Moon to make a complete rotation around the Earth. Because of this difference in rotation speeds, the tidal bulge created by the Moon on Earth actually rotates ahead of the Moon in its orbit. Earth s bulge pulls the Moon forward. This effect increases the Moon's energy. The increase in energy allows the Moon to pull further away from Earth and increase its orbital distance. As its distance increases, the Moon's orbital speed decreases according to the law of conservation of angular momentum.

16 Effects of tidal forces Estimates of the rate at which the Moon's orbit is increasing obtained by studying fossil deposits left by the tides over long periods of time. A more accurate measurement is also available thanks to Apollo astronauts who placed corner cube reflectors on the lunar surface in the 1970s. By bouncing laser beams off these reflectors, scientists can obtain a very precise distance to the Moon. Both methods have shown that the Moon's orbit is increasing by 3.8 cm per year. The fossil records further indicate that this rate has remained nearly unchanged for the past 900 million years. Within 500 million to one billion years, however, the Moon will have moved far enough from Earth that total eclipses of the Sun will no longer be possible for observers here on Earth.

17 Solar Eclipses Distance from Earth to the Moon is 384,403 km, distance from the Earth to the Sun is 149,597,887 km. If you divide these two numbers, you get 389. Diameter of the Sun is 1.4 million km, diameter of the Moon is 3,474 km, dividing gives 403. Those two numbers are pretty close. This is why the Moon and the Sun appear to be the same size in the sky. Because they appear to be the same size in the sky, the Sun, Earth and Moon work together to create eclipses.

18 Solar Eclipse!

19 Predicting New Moon 3 ingredients: Starting point and velocity (measurements) Physics - gravitation Integration - extrapolating into the future Most accurate ephemerides correct to seconds

20 Predicting Crescent Visibility New Moon calculations accurate to seconds. Record observation, 15.4hrs (1871, Athens) "Records of Young Moon Sightings, Quarterly Journal of Royal Astronomical Society (1993) 34, 53-56, article by Schaefer, Ahmad and Doggett In recent times crescents seen with naked eye have an angle of 10.5 relative to the Sun which corresponds to 17 to 21 hours of age.

21 Environmental factors affecting visibility Sky brightness (time) Dust and air pollutants Humidity Cloud cover Light pollution Observer experience Yallop Algorithm, Dr Bernard Yallop, HMNAO, 1997

22 Yallop s Algorithm Lunar visibility prediction taking account of: angle subtended at the centre of the Earth between centre of Sun and centre of Moon (width of crescent), geocentric difference in altitude at a given location, difference in azimuth at a given location. The best time is defined as the time of sunset increased by 4/9ths of the time interval between sunset and moonset. For instance, if the local times of sunset and moonset are at 18:00 and 19:30, then the best time to search for the lunar crescent is around 18:40. [twilight vs altitude/extinction] From an analysis of nearly 300 observations (successful or not) of the lunar crescent, Yallop determined a visibility criteria for the lunar crescent q

23 Yallop s Algorithm q > easily visible to the unaided eye < q < visible to the unaided eye under perfect atmospheric conditions < q < optical aid needed before seen with the unaided eye < q < only visible with binoculars or small conventional telescopes < q < only visible with large conventional telescopes q < below the Danjon limit (8 ) only visible with specialized telescopes using digital enhancement

24 Sunset CT17:52 - crescent 8hr 38m

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26 Sunset CT18:10 - crescent -5hr 40m

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29 Sunset CT18:52 - crescent 16hr 18m

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31 The Crescent Moon from Space