Understanding Solar and Lunar Eclipses: Types, Predicting and Observations, Study notes of Astronomy

Download Understanding Solar and Lunar Eclipses: Types, Predicting and Observations and more Study notes Astronomy in PDF only on Docsity! 1/25/05 Astro 121 Lecture 4, Prof. Mohr Solar and Lunar Eclipses Eclipses are the most dramatic astronomical events. 1/25/05 Astro 121 Lecture 4, Prof. Mohr Outline 1) Lunar Eclipses 2) Solar Eclipses 3) Predicting Eclipses 1/25/05 Astro 121 Lecture 4, Prof. Mohr Penumbra versus Umbra The darkest core of the Earth’s shadow, where no Sun is visible, is called the umbra Surrounding portion of Earth’s shadow, where only a part of the Sun is visible, is called the penumbra Figure from Seeds, The Solar System 1/25/05 Astro 121 Lecture 4, Prof. Mohr Progress of an Eclipse An eclipse begins as the Moon enters the Earth’s shadow from the right side, as viewed from Earth. Progression  Moon travels first into the penumbra  Then into the umbra  Then back into the penumbra  Eclipse ends as Moon exits Umbra Penumbra This process can take up to 6hrs! 1/25/05 Astro 121 Lecture 4, Prof. Mohr Because the Moon’s orbit is tilted 5o with respect to the ecliptic, sometimes the Moon passes above or below the Earth’s shadow  If this weren’t the case, we’d get a Lunar eclipse at every Full Moon!  Moon is 0.5o across, umbra is 1.3o, and the penumbra is 2.3o  Moon can appear anywhere between 5o above or 5o below the Earth’s shadow at the moment it is Full Total, Partial and Penumbral Eclipses Umbra Penumbra Total eclipse: fully into the umbra Partial eclipse: only partially into the umbra Penumbral eclipse: enters penumbra but never into the umbra +5o -5o +0o 1/25/05 Astro 121 Lecture 4, Prof. Mohr The Angular Diameter of the Moon and Sun By chance, our Moon and our Sun have almost identical angular diameters.  If the Moon were a bit smaller or a bit more distant, total solar eclipses would be impossible What is an angular diameter?  It’s the apparent size of an object when viewed from a distance Linear DiameterL Distance D Angular Diameter θ ! "[radians] = L D ! "[degrees] = L D 180o # 1/25/05 Astro 121 Lecture 4, Prof. Mohr Moon Shadow, Moon Shadow The Moon’s shadow (umbra) is a circle 269km in diameter on the Earth  So, the Moon’s shadow covers only a tiny spot on the Earth’s surface, whereas the Earth’s shadow is almost three times larger than the Moon  The Moon’s shadow moves across the Earth’s surface with a speed of 1700km/hr In contrast to the 6hr long Lunar eclipse, a Solar eclipse lasts only a couple of hours. Total eclipse lasts only 2 or 3 minutes! Whereas the Lunar eclipse may be viewed from anywhere on the dark side of the Earth, the Solar eclipse can be viewed from only a small part of the Earth 1/25/05 Astro 121 Lecture 4, Prof. Mohr Progress of a Solar Eclipse Stages of solar eclipse a) Moon moves from right into path of light from Sun to Earth b) Moon continues into light path, taking a bite out of the light coming from the Sun c) If total eclipse, then eventually the Moon fully covers the Sun. This period typically lasts only 2 to 3 minutes Figure from Seeds, The Solar System 1/25/05 Astro 121 Lecture 4, Prof. Mohr Annular Solar Eclipse The Moon’s and Earth’s orbits are slightly elliptical or non-circular. The Moon’s distance from Earth varies by ~12%, and the Earth’s distance from the Sun varies by ~3%. These variations alter the angular size of the Moon and Sun because ! "[radians] = L D 1/25/05 Astro 121 Lecture 4, Prof. Mohr 3) Predicting Eclipses We can understand the timing of Solar and Lunar eclipses by considering the Moon’s orbit relative to the ecliptic Figure from Seeds, The Solar System 1/25/05 Astro 121 Lecture 4, Prof. Mohr Moon’s Orbit and the Ecliptic The Moon’s orbit is tilted 5o with respect to the ecliptic.  Eclipses occur at New and Full Moon only if the Moon is crossing the ecliptic!  A point on the sky where the Moon crosses the ecliptic is called a node. There are two nodes where the Moon’s orbit crosses the ecliptic. The line connecting these two nodes is called the line of nodes. EarthSun Lunar eclipse Solar eclipse No eclipse No eclipse 1/25/05 Astro 121 Lecture 4, Prof. Mohr Eclipses and the Line of Nodes Figure from 21st Century Astronomy 1/25/05 Astro 121 Lecture 4, Prof. Mohr Moon’s Orbital Precession Moon’s orbit precesses, much as the rotation axis of the Earth precesses.  The Moon’s orbit precesses through an entire cycle every 18.6 years, as opposed to the Earth’s precession period of 26,000 years  The Moon’s orbital precession moves the line of nodes westward ~20o every year, so that rather than 6 months from one eclipse season to the next, it is more like 5 months and 3 weeks from one eclipse season to the next.  Amazingly, these patterns were discerned by many ancient cultures and used to make accurate predictions of Lunar and Solar eclipses.  See discussion of the Saros Cycle on pg 21 1/25/05 Astro 121 Lecture 4, Prof. Mohr Announcements Read first half of Chapter 2, Astronomy Today for next class “Origin of Modern Astronomy”