Understanding Lunar and Solar Eclipses: Phases of the Moon and Causes of Eclipses - Prof. , Study notes of Astronomy

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ASTR 111 – 003 Fall 2006 Lecture 03 Sep. 18, 2006 Introducing Astronomy (chap. 1-6) Introduction To Modern Astronomy II Ch1: Astronomy and the Universe Ch2: Knowing the Heavens Ch3: Eclipses and the Motion of the Moon Ch4: Gravitation and the Waltz of the Planets Ch5: The Nature of Light Ch6: Optics and Telescope Planets and Moons (chap. 7-17) Eclipses and the Motion of the Moon Chapter Three The phases of the Moon: causes • The phases of the Moon occur because – we see the varying amount of the illuminated half of the Moon, as the Moon orbits around the Earth – light from the Moon is actually reflected sunlight – At any moment, the Sun illuminates one half of the Moon • A new moon occurs, when the Moon is between the Sun and the Earth • A full moon occurs, when the Moon and the Sun are on the opposite side of the Earth Synchronous Rotation of Moon • Observations show that the Moon always keeps the same hemisphere, or face, toward the Earth. • Synchronous rotation: the Moon makes one rotation in exactly the same time that it makes one orbit around the Earth. Thus we only see the same face. • Synodic month (or lunar month): the Moon completes one cycle of phases, or one complete orbit around the Earth with respect to the Sun, averaging 29.53 days. • Sidereal month: the Moon completes one orbit around the Earth with respect to the stars, averaging 27.32 days. • The synodic month is longer, because – After the Moon travels 360° along its orbit around the Earth, the Earth has also traveled about 27° along its orbit around the Sun – To complete a cycle of phases, the Moon must travel the additional 27° along its orbit around the Earth, which takes about 2 days more Synodic Month and Sidereal Month Solar and Lunar Eclipses • Eclipses occur – when the Sun, Earth and Moon all happen to lie along a straight line, the shadow of Earth (Moon) falls on the Moon (Earth) • Lunar eclipse: the Moon passes through the Earth’s shadow – The Earth is between the Sun and the Moon – The Moon is at full phase – The full moon appears quite dim during lunar eclipse • Solar eclipse: the Earth passes through the Moon’s shadow – The Moon is between the Sun and the Earth – The Moon is at new phase – The Sun sometimes fully disappears in the clear sky during the solar eclipse. Eclipses do not Occur Every Month • The plane of the Moon’s orbit is tilted about 5° with respect to the plane of the Earth’s orbit (so called ecliptic plane) • At new and full phases, the Sun, Earth and Moon are often not along a straight line. • There are a few solar and lunar eclipses per year. • The maximum number (combined) in a single year is seven Eclipses and Line of Nodes • Line of nodes: the line along which the plane of the Moon’s orbit intersects the plane of the Earth orbit • Eclipses occur only when the Sun and Moon are both on the line of nodes • Or when the Moon is on the ecliptic plane at the time of new phase or full phase. Lunar eclipses • Totality: the period when the Moon is completely within the Earth’s umbra, which is a few times larger than the size of the Moon – Totality can last as long as 1 hour and 42 minutes. • A lunar eclipse can be seen at any place on Earth where it is nighttime. Solar eclipses • Total solar eclipse • Partial solar eclipse • Annular solar eclipse Total solar eclipse Annular solar eclipse Solar eclipses • The angular diameter of the Moon is almost the same as the angular diameter of the Earth – about 0.5° • Total solar eclipse: – Completely blocked by the Moon – Seen by people inside the umbra of the Moon’s shadow • Partial solar eclipse: – Only part of the Sun blocked by the Moon – Seen by people inside the penumbra of the shadow • Annular eclipse: – A thin ring of the Sun is seen around the edge of the Moon’s shadow – This happens if the Moon is at or near apogee; the Moon appears too small to cover the Sun completely Solar eclipses table 3-2 | Solar Eclipses, 2004-2008 2004 April 19 2004 October 14 2005 April 8 2005 October 3 2006 March 29 2006 September 22 2007 March 19 2007 September 11 2008 February 7 2008 August 1 Partial Partial Annular and Total Annular Total Annular Partial Partial Annular Total Antarctica, southern Africa Northeast Asia, Hawaii, Alaska New Zealand, North and South America Europe, Africa, southern Asia Africa, Europe, western Asia South America, western Africa, Antarctica Asia, Alaska South America, Antarctica Antarctica, eastern Australia, New Zealand Northeast North America, Europe, Asia 74% eclipsed 93% eclipsed Annular along part of path; maximum duration of totality Om 42s Maximum duration of totality 4m 7s 87% eclipsed 75% eclipsed Maximum duration of totality 2m 27s Eclipse predictions by Fred Espenak, NASA/Goddard Space Flight Center. All dates are given in standard astronomical format: year, month, day. Final Notes on Chap. 3 • Section 3-6 (on ancient astronomers and size of the Earth) is not taught.