Bringing Astronomy
Down to Earth
What do we know about
Earth – Moon – Sun
interactions?
Earth’s rotation

The Earth
rotates on
its axis (it
makes one
complete
turn) every
24 hours
Earth’s rotation is the cause of our
day and night
Which way does the Earth rotate?
From west to east
Earth’s revolution


The Earth
revolves around
the sun (makes
one complete
circuit) once
every 365.26
days
This is the cause
of our year
Leap year


Every 4 years we
add one day to the
calendar year to
make up the 1/4th
of a day that we
miss each year
The last leap year
was 2008, the next
will be _____.
Other variations in Earth’s
movement


Every 100,000
years the Earth’s
elliptical orbit
varies from more
elongated to more
circular
Every 41,000 years
the tilt of the Earth
on its axis varies
between 21.50 and
24.50
Precession: every 26,000 years
the circular motion of the Earth on its
axis...causes the Earth to spin like a
top on a table


Right now the axis
points towards
Polaris (North Star)
In the future will
point to the star
Vega
These are called Milankovitch
cycles and they are believed to be
one of the causes that the Earth’s
climate cycles between ice ages
and warm periods
What do we mean by latitude and
longitude?
Latitude = Horizontal (N or S)
Longitude = Vertical (E or W)
Lines of Latitude
00 to 900 N, 00 to 900 S
Important Lines of Latitude
What is the latitude of
Narragansett, RI?
http://www.epodunk.com/cgibin/genInfo.php?locIndex=13438
Aphelion/Perihelion


Because of the Earth’s elliptical orbit, the
distance between the earth and the sun
varies through the year
We are closest at perihelion; we are
furthest apart at aphelion (apart/aphelion)
Aphelion (furthest from sun) occurs
on July 4th (152 million km) and
Perihelion (closest to sun) occurs on
January 3rd (147 million km)
Closest in
January???
Then why is
it coldest???
What causes the seasons??
The tilt of the earth on its axis!!
During our summer: Northern hemisphere
fully faces the sun
During our winter: Southern hemisphere fully
faces the sun
A: Summer Solstice – June 20 or 21
solstice: means “sun” “stop” – the sun stops getting
higher in the sky each day
The sun’s rays are directed at the Tropic of Cancer
B: Autumnal
Equinox –
September 22 or 23
Equinox: means
“equal nights”;
length of day and
night are equal all
over the world
The sun’s rays are
directed at the
equator
C: Winter Solstice – Dec. 21 or 22;
sun is lowest in the sky
The sun’s rays are directed at the Tropic of
Capricorn
D: Spring
Equinox – March
20 or 21
The sun’s rays
are directed at
the equator
Daylight Change by Latitude
Monthly Variation in Daylight in Three
Cities
30
25
15
10
5
0
Ja
n
F uar
eb y
ru
ar
y
M
ar
ch
A
pr
il
M
ay
Ju
ne
Ju
A ly
S ug
ep u
te st
m
b
O er
ct
N ob
ov er
e
D mb
ec e
em r
be
r
Daylight Hours
20
M onth
Narragansett
Tromsö
Mexico City
Can you
detect the
times of
equinox?
Can you
detect the
summer and
winter
solstices?
What is an eclipse?
An eclipse occurs when one planetary body
passes through the shadow of another.
Eclipses are named depending on which
body is eclipsed, or blocked
Solar Eclipse: (the sun is eclipsed or blocked)
The moon is between the sun and earth - the
shadow of the moon falls on the earth
Within the umbra, you see a total eclipse (smaller area in
shadow)
Within the penumbra, you see a partial eclipse (larger
area in the shadow)
Solar eclipses never last > 7 minutes at any location.
Why??
Lunar Eclipse:
(the moon is eclipsed, or blocked)
The earth is between the moon and the sun - the
earth’s shadow falls on the lighted half of the
moon
Total lunar eclipse: moon must pass entirely
through the earth’s umbra
Partial lunar eclipse: only part of the moon passes
through umbra
Lunar eclipses may last hours!!
The moon looks reddish during a lunar eclipse
because the sunlight reflected off the moon is bent
in the earth’s atmosphere
Moon Phases
Phases of the Moon:
As Moon revolves around the Earth,
different parts of the lighted side of the
moon face Earth






New moon: moon is between sun and
earth – no visible moon
Full moon: earth is between sun and
moon – fully visible moon
Crescent moon: < half of moon visible
Gibbous moon: > half of moon visible
Waxing: visible part is increasing
Waning: visible part is decreasing
Moon Phases
New moon → waxing crescent → first quarter →
waxing gibbous → full moon → waning gibbous →
last quarter → waning crescent → back to new
moon
Phases of the Moon
Tides: daily changes in the level of
the ocean surface

Caused by
the
gravitational
pull of the
moon (and
the sun) on
the Earth’s
waters
Both Sun and Moon
cause the tides
Spring tides: S-E-M
Neap Tides: S-E
M
Spring tides: highest high tides of the month; at times of new and
full moons
Neap tides: lowest high tides of the month; at 1st and last
quarters
Neap and Spring Tides
Tides – 3 tidal patterns



diurnal tides: one
high, one low every 24
hours
semidiurnal tides: two
equal high, two equal
low every 24 hours
semidiurnal mixed
tides: 2 highs of
different heights, 2
lows of different
heights every 24
hours



Tidal range:
difference in height
between the
highest high and
the lowest low tide
Tidal period: time
between one high
tide and next high
tide
Tidal day: time
between 1st high
tide and last high
tide in 24 hours
What kind of tides in Eastport, ME?
Look at the daily tidal range!
What kind of tides in Grand Isle, LA?
Look at the daily tidal range
What kind of tides in
Crescent City, CA?
What kind of tides in Newport, RI ?
Look at the daily tidal range