PHYS 1403 Stars and Galaxies Topics for Today s Class Chapter 3: Lunar Phases Chapter 3: Lunar Eclipse Chapter 3: Solar Eclipse The Phases of the Moon Why we see Phases of Moon From Earth, we see different portions of the Moon s surface lit by the sun, causing the phases of the Moon. Pbs.org Full Moon rising on date shown Moon rising the next day 1
Consecutive Moon Rise Time Difference 2017/01/12: 18:32:00 2017/01/13: 19:36:00 Moon rises about one hour later every day We can use this information to estimate moon rise and moon set for any day The Phases of the Moon Evening Sky New Moon First Quarter Full Moon The Phases of the Moon Morning Sky Full Moon Third Quarter New Moon Lunar Eclipse In cultures all around the world, the Moon is a regular counter of the passing days and months Sometimes unusual events of the Moon arise 2
Earth s Shadow The shadow cast by a map tack can be used to understand how shadows produce eclipses Umbra = The region of total shadow Penumbra = The region of partial shadow The Angular Diameters of the Sun & Moon Penumbra = The region of partial shadow Small Angle Formula angular diameter linear diameter 206,265 distance Example for Moon angular diameter 206,26 5 3476 km 384,000 km 1870 angular diameter 187 0 0. 5 60 60 Angular Diameter of Sun and Moon is important for Eclipses to occur Lunar Eclipses Earth s shadow consists of a zone of partial shadow, the Penumbra, and a zone of full shadow, the Umbra. If the moon passes through Earth s full shadow (Umbra), we see a lunar eclipse. If the entire surface of the moon enters the Umbra, the lunar eclipse is total. Types of Lunar Eclipses The Moon s orbit is inclined by a bit more than 5º relative to Earth s orbital plane Moon does not always pass through center of the Umbra Total lunar eclipse All of Moon enters the umbra Partial lunar eclipse Some part of the moon is in Umbra Rest of the Moon enters the penumbra Penumbral lunar eclipse Moon only enters the penumbra Moon is only partially dimmed A Total Lunar Eclipse (1) 3
A Total Lunar Eclipse (2) A total lunar eclipse can last up to 1 hour and 40 min. During a total eclipse, the moon has a faint, red glow, reflecting sun light scattered in Earth s atmosphere. Lunar Eclipses: 2002-2012 Typically, 1 or 2 lunar eclipses per year. Solar Eclipses For millennia, cultures worldwide have understood that the Sun is the source of life People explained the terrifying sight of the Sun gradually disappearing with stories Observing an Eclipse Solar Eclipses The sun appears approx. as large in the sky (same angular diameter ~ 0.5 0 ) as the moon. When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse. 4
The Moon s Shadow: A Total Solar Eclipse Solar Eclipses: 2002-2012 Approximately 1-2 total solar eclipse per year (a) The umbra of the Moon s shadow sweeps from west to east across Earth, and observers in the path of totality see a total solar eclipse. Those outside the umbra but inside the penumbra see a partial eclipse. (b) Eight photos made by a weather satellite have been combined to show the Moon s shadow moving across the eastern Pacific, Mexico, Central America, and Brazil during an eclipse in 1991. August 2017 Solar Eclipse in US Total Solar Eclipse Chromosphere and Corona https://eclipse.gsfc.nasa.gov/segoogle/segoogle2001/se2017aug21tgoogle.html Prominences NASA Solar Atmosphere Revealed Diamond Ring Effect 5
Earth and Moon s Orbits Are Slightly Elliptical Perihelion = position closest to the sun Sun Earth Perigee = position closest to Earth (Eccentricities greatly exaggerated!) Apogee = position furthest away from Earth Moon Aphelion = position furthest away from the sun Annular Solar Eclipses When Earth is near perihelion, and the moon is near apogee, we see an annular solar eclipse. Perigee Apogee Perihelion Aphelion The angular sizes of the moon and the sun vary, depending on their distance from Earth. Annular Solar Eclipses (2) Predicting Eclipses Astronomers in early civilizations could make educated guesses as to which full moons and which new moons might result in eclipses Making exact eclipse predictions requires a computer and proper software Today we can predict precise date and time of all eclipses Almost total, annular eclipse of May 30, 1984 Lunar Node The moon s orbit is inclined against the ecliptic by ~ 5 0. A points where the lunar orbit intersects the ecliptic is called a node. Conditions for Eclipses Eclipses occur only near lunar nodes Ecliptic Wikipedia A solar eclipse can only occur if the moon passes a node near new moon. A lunar eclipse can only occur if the moon passes a node near full moon. 6
Conditions for Eclipses Line of Nodes has to point towards the Sun Saros (Eclipse) Cycle The Eclipse cycle is a period of 18 years, 11 days and 8 hours (6585.321 days or 223 lunar months) From the first eclipse of the year the line of nodes slips westward 19.4 degrees per year. Causing the eclipse season to begin 19 days earlier every year. Eclipses occur in a cyclic pattern. Saros Cycle Solar and Lunar Eclipse https://www.youtube.com/watch?v=kgbk2fzffdw https://www.youtube.com/watch?v=is8olhgglae After 3 Saros cycles (54 years 1 month). The eclipse cycle occurs in the same part of the earth www.astronomy.com Acknowledgment The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class. 7