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Unit 2 – The Moon and the Planets ASTR 101 Prof. Dave Hanes The Moon’s Baleful Influence (?) Crime rates, homicides, birth rates, acts of‘lunacy,’ traffic accidents, etc… http://skepdic.com/fullmoon.html The Apparent Size of the Moon Lunar Phases Not caused by the Earth’s shadow! The phases arise because we see the Moon from different perspectives as it orbits the Earth, and can see only part of the half of the Moon that is lit up by the Sun. Watch this very helpful video for a clear understanding: https://www.youtube.com/watch?v=wz01pTvuMa0 Full Moon For the moon to be full, it has to be on the opposite side of the sky from the Sun (so we can see its fully lit-up face). This means that it will be rising in the East as the Sun is going down in the West. It will be ‘up’ all night. Thin Crescent We see a thin crescent only when the Moon is close to the Sun in the sky. (If big enough, it can be seen in the daytime sky.) If it’s to the left (East) of the Sun, it sets in the West shortly after the Sun itself does (and rises after the Sun). If it’s to the right (West) of the Sun, it sets in the West shortly before the Sun does (and rises before the Sun). The Moon’s Neverchanging Face The Moon rotates exactly once every time it goes around the Earth. This is a ‘resonance’ – not a coincidence! – to be explained later. Any astronauts on the front of the Moon would always see the Earth. No Dark Side The far side of the moon is hidden from Earth -but it is not always in darkness! It has alternating light and dark, on a one-month timescale. Angular Size How big an object looks (its angular size) depends on two things: How big it really is (its linear size); and How far away it is. (Even a small object can look big if it is close to you.) An Amazing Coincidence The Sun and the Moon have almost exactly the same angular size. The Sun is 400x as big, but also 400x farther away! This means that when the Moon ‘gets in the way,’ we see perfect eclipses: the Moon only just blocks off the Sun. A Total Solar Eclipse (Lets us see the faint, distended corona of the Sun) Thinking About Shadows Umbra vs Penumbra Everything is Moving! The dark umbral shadow sweeps across the land. The longest possible period of totality is about 7 minutes. A Recent Total Solar Eclipse 20 March 2015 Simulation prepared by NASA: https://www.youtube.com/watch?v=EpDlbbpc8GA Note: The big penumbral shadow. People here see a partial eclipse The smaller dark umbra. When this passes over you, you experience a total eclipse. (The maximum possible duration is ~7 minutes.) Many people on Earth see nothing special at all! Annular Eclipse (an “annulus” is a ring.) This happens when the moon is a bit farther away from the Earth than usual, so can’t block the whole face of the Sun. Annular Eclipse Lunar Eclipses The Earth casts a big shadow, through which the Moon can pass. Unlike solar eclipses, everyone can see this! -- no need to be in a special location. Surprise! The Moon Does Not Vanish (even though it passes right through the Earth’s umbra) The Moon gets dim, and turns reddish. Why? Sunlight is Scattered by the Earth’s Atmosphere: Some Still Reaches the Moon Eclipses are Rare! (at most a few a year) Why is there not a solar eclipse every month at the time of New Moon, when they are ‘lined up’? It is because the Moon’s orbit is not in exactly the same plane as the Earth’s orbit around the Sun. To understand this fully, watch https://www.youtube.com/watch?v=X6HAp2ft0NE Clockwork! The orbits of the Sun, Moon and Earth are like gears in a a complex clock. The same arrangement will recur at regular intervals. Consequently, eclipses repeat (in very similar form) after a period of about 54 years. This is just a couple of human generations, so the ancients learned to predict them! Two Similar Eclipses from computer simulations Kingston, 8 April 2024 http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Eclipse-Apr2024.mov Kingston, 11 May 2078 http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Eclipse-May2078.mov Rare Events – But Just Wait! The Distance and Size of the Moon Once we know its distance, we can determine its true size! Here’s a scale model showing the relative sizes (not the real separation!) A Crude Argument (used by the ancient Greeks) During a lunar eclipse, assume that the Earth’s shadow (projected on the Moon) is about as big as the Earth itself. Since they knew the size of the Earth (from Eratosthenes), they could then work out: the approximate size of the moon, and thus deduce its approximate distance More Precisely: Use Parallax Measurements Analogy: Binocular Vision Our two eyes see two different images, giving depth perception Meet Aldebaran Starry Night Sept 4, 10:00 PM – watch the moon from Kingston. It occults Aldebaran as it drifts across the field of background stars. (Do this with your Starry Night software!) Same date and time: from Buenos Aries, we do not see this! Why? Meet The Pleiades (a very remote star cluster) In glorious colour More prosaically, as photographed by an astronomer The Moon Occasionally Passes in Front - for instance, 29 Oct 2004 It looks different from different places, even at the very same instant! (a parallax effect) Kingston Santiago, Chile Why? K S Remember that each star in the Pleiades is enormously larger than the Earth + Moon, and that they are fantastically far away. Indeed, the star cluster is so very remote that we look towards it in parallel directions from Kingston (K) and Santiago (S), as shown by the arrows. (The stars are way off to the upper right , not as drawn in this figure. ) From Kingston, the Moon lies right in front of the cluster. From Santiago, we have to shift our gaze a little to one side to see the Moon – a parallax effect. Measuring the angle through which we shift our gaze, and knowing the distance between K and S, we can figure out the distance to the Moon! The Most Precise Distance Determination Bounce radar signals or laser light off the surface (where the Apollo astronauts left special reflectors). See how long it takes for the return signal to reach you. This is very precise! Now Consider the Planets Two categories: Inferior planets (closer to the Sun than we are): Mercury and Venus Superior planets (farther out): Mars, Jupiter, Saturn, Uranus, Neptune. Inferior Planets Inferior Planets They are always fairly close to the Sun: sometimes to the left (East) of it, visible in the evening sky; sometimes to the right (West) of it, visible in the dawn sky. They come and go at times as the planets orbit the Sun. They can never be seen overhead at midnight. Transits Inferior planets can occasionally pass between us and the Sun, and be seen in silhouette Mercury Venus Superior Planets They can be overhead at midnight (at “opposition”), or around on the far side of the sun (at “conjunction”) - or at any intermediate point in their orbits. But they never pass between us and the Sun. Motions of the Planets Thanks to its orbit around the Earth, the Moon drifts continuously to the East across the background field of stars. The superior planets move in big orbits around the Sun, so you might expect them to behave in roughly the same way. Instead, but they do something quite unexpected. ‘Retrograde’ Motion As we watch a superior planet like Mars over the space of several weeks or months, its steady Eastward drift slows and comes to a halt, after which it moves backwards for a time. Eventually, it stops again, and Mars resumes its original Eastward drift. See http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Mars-Ret.mp4 This is a bizarre complexity we will come back to. It is an important clue to the structure of the Solar System.