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Exploring the Solar System Lecture 3: Structure of the Moon Professor Paul Sellin Department of Physics University of Surrey Guildford UK Page 1 Paul Sellin Lecture 3 Page 1 Overview Physical properties of the Moon Rotation and phases of the Moon The Moon’s orbit Observation of the Moon’s features from Earth Formation of the Moon Crater formation and the lunar regolith Page 2 Paul Sellin Lecture 3 Page 2 Page 3 Paul Sellin Harrison Schmidt walking on the lunar surface – December 1972. Photographed by Eugene Cernan, Apollo 17. This was the last manned mission to the Moon. The Moon has only been visited by 12 people. The large boulder in the image is one of the most recent surface features on the Moon – it tumbled down a mountainside to its present location a mere 800,000 year ago. Unlike Earth, the surface of the Moon has remained essentially unaltered for billions of years. Lecture 3 Page 3 Guiding Questions What key findings resulted from the manned exploration of the Moon during the Apollo program in the 1970s? Does the Moon’s interior have a similar structure to the interior of the Earth? How do Moon rocks compare to rocks found on the Earth? How did the Moon form? The image shows the Earth and Moon to scale, taken from the Galileo spacecraft Both objects are at the same distance of 6.2 million kilometres from the spacecraft The moon shows no atmosphere, no oceans, and no evidence of plate tectonics Page 4 Paul Sellin Lecture 3 Page 4 Size of the Moon Compared to planetary distances, the Moon is very close – just 384,400 km The angular diameter is only 1/2° the diameter of the moon is relatively small The orbital motion of the Moon around the Earth cannot be considered as simple rotation of an object around a fixed point: mass of the Moon is 1.23% of the mass of the Earth both objects orbit around a fixed centre of mass of the Earth-Moon system – this is inside the volume of the Earth. The centre of mass moves in an elliptical orbit around the Sun: The actual value for the diameter of the moon is 3476 km, 27% of the Earth’s diameter Page 5 Paul Sellin Lecture 3 Page 5 Page 6 Paul Sellin Lecture 3 Page 6 Phases of the Moon Page 7 Paul Sellin The figure shows the Moon at 8 positions in its orbit, plus photographs of what the moon look like at each position as seen from Earth. The changes in phase occur because light from the Sun illuminates one half of the Moon, and as the Moon orbits the Earth we see varying amounts of the Moon’s illuminated half. It takes 29.5 days (synodic period) to go through a complete cycle of phases. Note that the phases of the moon are not due to the shadow of the Earth falling on the Moon’s surface – this is a popular misconception. When this occurs we get as lunar eclipse. Notice that the Moon does rotate around its axis – see next slide Lecture 3 Page 7 Rotation of the Moon The phase of the Moon is always changing, but the Moon always keeps the same hemisphere towards the Earth the Moon rotates about its axis with the same period as its orbit around the Earth: Page 8 Paul Sellin Lecture 3 Page 8 The Dark Side of the Moon “There is no dark side of the moon… matter of fact it’s all dark” So said the final line of Pink Floyd’s famous 1973 concept album ‘Dark Side of the Moon’ Pink Floyd, clockwise from the top: Roger Waters, Nick Mason, Dave Gilmour, Rick Wright. Page 9 Paul Sellin Lecture 3 Page 9 The real ‘dark side of the moon’ – as seen by Apollo 16 Locked in synchronous rotation, the Moon always presents its well-known near side to Earth. But from lunar orbit, Apollo astronauts also observed the Moon's far side. This sharp picture from Apollo 16's mapping camera shows the eastern edge of the familiar near side (left) and the heavily cratered far side of the Moon. Surprisingly, the rough and battered surface of the far side looks very different from the near side which is covered with smooth dark lunar maria. The likely explanation is that the far side crust is thicker, making it harder for molten material from the interior to flow to the surface and form the smooth maria. Page 10 Paul Sellin Lecture 3 Page 10 Both sides of the Moon Photo: our back garden, ~1960 Photo: NASA, Apollo 16, 1972 Page 11 Paul Sellin Lecture 3 Page 11 Tidal forces (1)* In the Earth-Moon system, tidal forces are the gravitational forces between these two bodies. The near and far sides of the Moon will experience different gravitational force, due to the varying distances Page 12 Paul Sellin Lecture 3 Page 12 Tidal forces (2)* The tidal force experienced by the Moon, due to Earth, is proportional to R3, such that: Problem: calculate the tidal force for two 1kg rocks on either side of the Moon’s surface G = 6.67x10-11 N m2/kg2 M = 6x1024 kg d = 3476 km R = 384,400 km Page 13 Paul Sellin Lecture 3 Page 13 Tidal forces (3)* Now calculate the gravitational force experienced by a 1kg mass on the Moon’s surface: Thus the tidal force on the lunar surface is much less than the gravitational force But the tidal force due to Earth deforms the Moon’s surface, producing tidal bulges around the equatorial region These tidal forces produce a torque on the Moon which keeps the rotation locked into synchronisation with the orbit around the Earth Page 14 Paul Sellin Lecture 3 Page 14 The Sidereal and Synodic Months The sidereal month is the time the Moon takes to complete one revolution of the Earth with respect to the background stars = 27.32 days Because the Earth is moving along its orbit, the Moon must travel through more than 360° to get from one new moon position to the next the synodic month is the time from one new moon to the next. This is longer than the sidereal month, = 29.53 days This is the period of the new moon and full moon Page 15 Paul Sellin The time for a complete lunar day (the time for the Moon to rotate once on its axis) is about 4 weeks. Because the Moon’s rotation is synchronous, it takes the same time for complete lunar orbit. Astronomers define two types of lunar month, depending on whether the Moon’s motion is measured relative to the stars or to the Sun: Sideral month – time for one complete orbit of the Earth, with respect to the stars (‘true’ orbital period) Synodic month, or lunar month – time for the Moon to complete one cycle of phases (eg. full moon to full moon), and hence return to the same position relative to the Sun. The synodic month is longer by about 2.2 days because the Earth is orbiting the Sun whilst the Moon goes through its phases. Thus the Moon must travel more than 360° around its orbit in order to complete a cycle of phases. Lecture 3 Page 15 The Moon’s Orbit The Moon’s orbit around the Earth is elliptical, not circular: The Moon is at perigee when it is nearest the Earth The Moon is at apogee when it is farthest from the Earth The line connecting these 2 points passes through the Earth and is called the line of apsides Average centre-centre distance of the Earth and Moon is 384,400 km Minimum distance is 356,410 km and maximum distance is 406,697 km Average speed of the Moon along its orbit is 1.02 km/s As seen from the Earth the Moon moves eastwards through the constellations from one day to the next: • the Moon’s daily eastern progress averages 13.2° (360°divided by 27.32 days in the sidereal month) •in 1 hour the moon moves slightly more than 1/2° which is a bit larger than its own diameter • moonrise is about 50 minutes later from one day to the next Page 16 Paul Sellin Lecture 3 Page 16 Inclination of the Moon’s orbit and the Line of Nodes The image shows the Moon’s orbit around the Earth (yellow) and part of the Earth’s orbit around the Sun (red) The plane of the Moon’s orbit (brown) is tilted by ~5° wrt the plane of the Earth’s orbit (blue, plane of the ecliptic) The 2 planes intersect along the line of nodes Lunar Eclipse An eclipse can only occur when the Sun and Moon are both very near to or on the line of nodes Only then can the Sun, Moon and Earth all lie along a straight line, required for the Moon’s shadow to fall on the Earth A solar eclipse occurs if the Moon is on the line of nodes at new moon A lunar eclipse occurs if the Moon is on the line of nodes at full moon Page 17 Paul Sellin Lecture 3 Page 17 Conditions for the Eclipses Page 18 Paul Sellin An eclipse requires that the Sun, Earth and Moon all fall on the same line. A lunar eclipse occurs when the Moon passes through the Earth’s shadow, ie. when the Earth is between the Sun and the Moon.
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