Transcript Document

The Earth, Sun, and Moon
STONEHENGE
Stonehenge
• Built 2800-1800 B.C.
•It’s the oldest timepiece in existence
• Designed for astronomical observation
•It’s a monument in S. England
• Helps in predicting eclipses & cycles of
moon
• Aligns at the solstices
Map of
Stonehenge
Stonehenge Astronomy
Diurnal Motion
• Geocentrially speaking, the
Earth sits inside a celestial
sphere. Fixed on the
sphere are stars.
• Each day, the Earth rotates
once (west-to-east) on its
axis. This causes us to face
different directions and see
different stars. The stars’
daily (diurnal) motion reflects
the Earth’s spin.
The Yearly Motion
In addition to rotating, the Earth also revolves about the Sun.
As the Earth revolves, the
Sun is projected in front
of different constellations
at different times of year.
The path the Sun takes
across heavens is called
the ecliptic. The
constellations which the
Sun passes through are
zodiac constellations.
Because the Sun is bright, we can only see some constellations at
certain times of year.
Motions of Earth
• Revolution is the motion of a body,
such as a planet or moon, along a path
around some point in space.
• Perihelion is the time in January when
Earth is closest to the sun.
• Aphelion is the time in July when Earth
is farthest from the sun.
Motions of Earth
• Earth’s Axis and Seasons
– The plane of the ecliptic is an imaginary
plane that connects Earth’s orbit with the
celestial sphere.
– Because of the inclination of Earth’s axis to
the plane of the ecliptic, Earth has its
yearly cycle of seasons.
The Seasons
Since the plane of the ecliptic is tilted 23.5° with respect
to the celestial equator, we have seasons.
The Motion of the Sun
From the Geocentric point of view, the Sun moves from the
northern part to the southern part of the sky with the seasons.
When the Sun is furthest north (south), it’s the summer (winter)
solstice. When the Sun crosses the celestial equator, it’s the
vernal (or autumnal) equinox.
The Day
Because the Earth revolves about the Sun, the word “day” has two
definitions.
A sidereal day is one
rotation with respect
to the stars.
A solar day is one
rotation with respect
to the Sun.
Sidereal and solar
days differ by about
4 minutes.
Precession
In addition to its rotation and revolution, the Earth’s axis also
precesses (wobbles) like a top. The angle between the ecliptic
and the equator remains at 23.5°, but the direction changes.
The period of this precession of the equinoxes is about 26,000
years.
Because of precession:
• Polaris won’t always
be the North Star
• The solar system
speeds in the direction
of the star Vega.
• Earth is presently
approaching one of its
nearest galactic
neighbors, the Great
Galaxy in Andromeda
The Lunar Period
The Moon moves west-toeast in the sky (like the
Sun), and takes about a
month to circle the Earth
(hence the word month).
But once again, there’s a
difference between the
Moon’s sidereal period
with respect to the stars
(27 days), and the synodic
period with respect to the
Sun (29 days).
Months are defined on the
basis of the moon’s
revolution around the
earth
• Perigee is the
point at which the
moon is closest to
Earth.
• Apogee is the
point at which the
moon is farthest
from Earth
Synodic Month: One lunar revolution from full moon to
full moon. Defined relative to the positions
of the earth and the moon.
Sidereal Month: One lunar revolution measured with respect to
the fixed stars.
Months and Years are not composed of even numbers of days.
Time Period
Year
Sidereal Month
Synodic Month
Length
365.2425 days
27 days, 7 hours, 43 minutes
29 days, 12 hours, 44 minutes
Phases of the
Moon
The Moon reflects light
from the Sun, so its
phase tells you the
relative position of the
Sun.
You can therefore tell
time from the Moon!
Phases of the
Moon
Full Moon occurs when
the Moon is opposite in
the sky from the Sun.
This is called opposition.
New Moon occurs when
the Moon is in the same
direction as the Sun, i.e.,
in conjunction.
The Moon’s
Shadow
The Moon’s orbital
plane is tilted by
5.2° from the
ecliptic plane.
Hence, ½ the time,
the Moon is slightly
north of the ecliptic
(and ½ the time, it
is south of the
ecliptic). The
shadow of one body
very rarely falls on
the other.
Lunar Eclipses
If the Moon crosses the ecliptic plane while exactly opposite the Sun, it
will fall in the Earth’s shadow. This is a lunar eclipse. This happens
roughly twice a year.
This is an animation showing a total lunar eclipse
from start to finish.
Solar Eclipses
If the Moon crosses the
ecliptic plane while
exactly in front of the
Sun, a tiny piece of the
Earth will fall in the
Moon’s shadow. This is
a solar eclipse. Since the
Moon and Sun appear to
be the same size in the
sky, the alignment of a
total solar eclipse must be
perfect.
Total Solar Eclipse
You only get to see this outer atmosphere (corona) of the Sun if
the Sun’s entire body is blocked out.
Here is an animation showing a total solar eclipse
• https://vimeo.com/122795016
Partial Solar Eclipse
If you’re on the edge of an eclipse path and only a slice of the Sun is
blocked out, it’s called a partial eclipse. These are not very exciting,
since the uneclipsed part of the Sun is still extremely bright.
Annular Eclipses
Because the Moon’s orbit
about the Earth is not
perfectly circular, it is
sometimes too far away to
block out the whole Sun,
even when perfectly aligned.
When this happens, you get
an annular eclipse. It is rarer
than a total solar eclipse, but,
like other partial eclipses,
they are, in general, pretty
boring.