Download Orbit - Geneva 304

The Moon
Orbit
 Very circular (eccentricity ~ 0.05)
 Orbital plane is tilted about 50
 Perigee - Moon is closest to the Earth
 Apogee - Moon is farthest from the Earth
Revolution
 Sidereal month - the Moon appears to move completely around the
celestial sphere (3600) ~27.3 days
 Synodic month - the time it takes for the Moon to return to the same
point on the celestial sphere with respect to the sun (>3600) ~29.5 days
Rotation
 The Moon has a rotational period of 27.3 days (coincides with it's sidereal
period) = tidal lock
 Because of this the Moon always keeps the same face pointed toward the
Earth
 The Earth's rotation is slowing and will come into tidal lock with the
Moon, and will eventually (in billions of years) face the Moon as it rotates
(same side will always face the Moon)
Tides
 Tides - caused by the Moon passing overhead - occur about 1 hour later
each day
 High tides occurs about 20 minutes after the Moon passes overhead, and
again about 12.5 hours later
 Every point on Earth is attracted to the
Moon; the greatest pull is located closest
to the Moon
 Water is pulled up slightly towards
the Moon = high tide (A)
 Land is pulled up slightly as one
whole unit towards the Moon, but
not as much as the water = high tide
on the opposite side of the Earth (C)
 Low tides fall in between (top and bottom)
 Spring and Neap Tides
o Other objects in the solar system influence the Earth's tides
o Sun affects our tides to a very small degree
o Spring tide - particular large tides that occur when the Sun and
Moon are lined up with the Earth at new and full Moon phases
o Neap tide - tides are
weakest when the
Moon is at first or last
quarter phase (at right
angles to the Earth
from the Sun); the
gravitational pull of the
Sun and the Moon on
the Earth's oceans
interfere with each
other
Phases
 The Moon is a sphere; one side is illuminated by the sun at all times
 The phase that we see from Earth depends on the positions of the Sun,
Moon, and Earth with respect to one another
 Moon phases
o New: non-illuminated surface, rises at dawn
o Waxing Crescent: only a sliver illuminated, rises in the late
morning
o First Quarter: half of the visible side lighted, rises about noon
o Waxing Gibbous: mostly lighted, rises in the late afternoon
o Full: visible side completely illuminated, rises at dusk
o Waning Gibbous: mostly lighted, rises in the late evening
o Third Quarter: half of the visible side lighted, rises at midnight
o Waning Crescent: only a sliver illuminated, rises after midnight
 Why doesn't the Moon
disappear when the Earth is
between it and the Sun?
 Moon is tilted 50 from the
ecliptic (the Earth's path
around the Sun)
 Planets go through phases
as well based on the same
principle
Eclipses
 Consequence of the Moon's orbit around the Earth is that the Moon
shadows the Sun's light from the Earth (solar eclipse) or the Moon passes
through the shadow of the Earth (lunar eclipse)
 This happens at the point in the Moon's orbit that crosses the ecliptic (the
Earth's orbit around the Sun)
 An eclipse of either type usually happens about 7 times a year experienced
somewhere on the Earth's surface
Solar Eclipses
 Alignment => Sun / Moon / Earth
 Moon must be at New Phase
 Geometry
o Umbra - darkest
part of shadow
o Penumbra intermediate
shadow
 Types of solar eclipses
o Total Solar Eclipse
 Umbra of the Moon's shadow touches a region (usually only a
few hundred kilometers across) on the Earth's Surface
 Partial Phase - Moon slowly (takes about an hour) covers the
Sun
 Duration of totality - lasts for a few seconds to about 7
minutes depending on the alignment of the eclipse
o Partial Solar Eclipse
 The penumbra of the Moon's shadow passes over a region on
the Earth
o Annular Solar Eclipse
 The Earth is in line with the umbra, but so far away that the
tip of the umbra does not reach the Earth
 Solar eclipses usually occur somewhere in the world every 1.5 years
 Next total solar eclipse in our area will occur August 21, 2017
Lunar Eclipses
 Moon passes through the
umbra of the Earth's
shadow
 Full Moon phase required
 Can be partial or total
 Atmospheric refraction causes red glow on the Moon