- Slides: 26
The Earth and the Moon • The Moon is our nearest neighbour. • It crosses the sky at ~12º per day, or its own diameter (~30 arc minutes) in ~ 1 hour
Lunar Phases • ~29. 5 days between new moons • Reflected light from the Sun • 3 -D system! Sphere, not discs.
Lunar Phases II
“Sidereal” and “Synodic” Months Due to earth’s motion around the Sun: the moon must travel slightly more than one full rotation to return to the same phase.
More Definitions and Observations II Sidereal means “with respect to the Stars”. Synodic means “with respect to the Earth”. • The sidereal month is is about 27 1/3 days. (i. e. the time for the moon to return to its original position relative to the fixed stars. ) • The synodic month (the time between successive new moons) is about 29 1/2 days.
More Definitions and Observations II • Twelve synodic months make about 360 days, or approximately one year. • 235 synodic months is almost exactly equal to 19 years (with an error of only two hours. ) This coincidence, together with the apparent similarity in the size of the sun and moon, serves to create the illusion that there is some fundamental connection between their motions. (It also enables the possibility of eclipses. )
The Moon’s Rotation • The Moon rotates on its own axis once per orbital cycle. • We therefore see the only one ‘face’ of the moon The ‘Dark Side of the Moon’ is a misnomer!
Orbital Planes of the Earth and Moon’s orbital plane is inclined at 5º to the plane of the ecliptic. The two planes intersect along the ‘Line of Nodes’.
Solar and Lunar Eclipses • Coincidentally, the angular sizes of the Sun and Moon are very similar. • This enables the possibility for eclipses!
Conditions for Eclipses Sun, Earth, and Moon need to be co-linear and nearly co -axial with the Line of nodes for eclipses to occur.
Lunar Eclipse: Earth Shadows the Moon is ‘reddish’ due to Earth’s atmosphere scattering sunlight.
Incidentally… • Full moons that occur when the moon is low in the sky (near the horizon) also appear red in colour. • Red colouration is an atmospheric effect… • It appears larger too. . “Harvest Moon” – the full moon nearest the autumnal equinox
Moon Illusion • The moon appears bigger near the horizon. • Our eyes/brains perceive it, cameras don’t! Debate as to true cause…. Must one have an horizon to see it? !
Eclipse Terms Note: The Sun has a finite size and therefore different ‘degrees’ of shadow exist. “Umbra” - Central region “Penumbra” - Annular ring ‘Partial’ ‘shadow/shade’
Total, Penumbral and Partial Lunar Eclipses Darkness depends on the degree of shadowing of the Sun.
Total Lunar Eclipse Sequence The Moon moving through the Earth’s umbra over a 3 hour period in January 2000.
Solar Eclipse (in 5 Minute Intervals) Sun’s corona in at the moment of total eclipse
View from Mir (1999) Dark spot is ~105 km wide moving at 3000 km/hr! Annular Solar Eclipse
Annular or Total Eclipse? Depends on the precise Earth-Moon distance at the time: Elliptical orbits.
Annular, Total and Partial Eclipses Depends on: (a) Earth-Moon distance. (b) Your position! (c) ‘New’ moon in ‘eclipse season’ i. e. depends on the ‘line of nodes’ º Total Eclipses are relatively rare.
What If ? ? If the Moon were larger…. annular eclipses would be rarer and totals more common. If the Sun were larger…. . never see total eclipses, only annular!
Eclipse Cycles • Sun-Moon gravitational interaction causes the ‘line of nodes’ to change orientation slowly with time. • 1 ‘Eclipse year’ = 346. 6 days. Time between one orbital configuration (with the line of nodes pointing at the Sun) and the next – with the Moon crossing the Ecliptic in the same manner.
‘Regression of the Line of Nodes’ The ~19 day difference from the Tropical year is known as the “Regression of the Line of Nodes”
Long-Term Eclipse Cycles • Combination of Synodic months (29. 5 days) and Eclipse years (346. 6 days) • 19 Eclipse years ~ 223 Lunar months = 6585 days (18 years 11. 3 days) • This is the 18 -year “Saros Cycle” …. . . Well-known to ancient Astronomers
Eclipses are Predictable! …and have been since the Egyptians and Babylonians!
Windsor April 8 2024 Saros Cycle