Bringing Astronomy Bringing Astronomy Down to EarthDown to Earth

What do we know about What do we know about

Earth – Moon – Sun Earth – Moon – Sun interactions?interactions?

Earth’s rotationEarth’s rotation

The Earth The Earth rotates on rotates on its axis (it its axis (it makes one makes one complete complete turn) every turn) every 24 hours24 hours

Earth’s rotation is the cause of our Earth’s rotation is the cause of our day and nightday and night

Which way does the Earth rotate?Which way does the Earth rotate?From west to eastFrom west to east

Earth’s revolutionEarth’s revolution The Earth The Earth

revolves around revolves around the sun (makes the sun (makes one complete one complete circuit) once circuit) once every 365.26 every 365.26 daysdays

This is the cause This is the cause of our of our yearyear

Leap yearLeap year

Every 4 years we Every 4 years we add one day to the add one day to the calendar year to calendar year to make up the 1/4make up the 1/4thth of a day that we of a day that we miss each yearmiss each year

The last leap year The last leap year was 2008, the next was 2008, the next will be _____.will be _____.

Other variations in Earth’s Other variations in Earth’s movementmovement

Every 100,000 Every 100,000 years the Earth’s years the Earth’s elliptical orbit elliptical orbit varies from more varies from more elongated to more elongated to more circularcircular

Every 41,000 years Every 41,000 years the tilt of the Earth the tilt of the Earth on its axis varies on its axis varies between 21.5between 21.500 and and 24.524.500

Precession: every 26,000 yearsPrecession: every 26,000 yearsthe circular motion of the Earth on its the circular motion of the Earth on its axis...causes the Earth to spin like a axis...causes the Earth to spin like a

top on a tabletop on a table Right now the axis Right now the axis

points towards points towards Polaris (North Star)Polaris (North Star)

In the future will In the future will point to the star point to the star Vega Vega

These are called These are called MilankovitchMilankovitch cycles and they are believed to be cycles and they are believed to be one of the causes that the Earth’s one of the causes that the Earth’s climate cycles between ice ages climate cycles between ice ages

and warm periodsand warm periods

What do we mean by latitude and What do we mean by latitude and longitude?longitude?

Latitude = Horizontal (N or S)Latitude = Horizontal (N or S)

Longitude = Vertical (E or W)Longitude = Vertical (E or W)

Lines of LatitudeLines of Latitude0000 to 90 to 9000 N, 0 N, 000 to 90 to 9000 S S

Important Lines of LatitudeImportant Lines of Latitude

What is the latitude of What is the latitude of Narragansett, RI?Narragansett, RI?

http://www.epodunk.com/cgi-bin/genInfo.php?locIndex=13438

Aphelion/PerihelionAphelion/Perihelion Because of the Earth’s elliptical orbit, the Because of the Earth’s elliptical orbit, the

distance between the earth and the sun distance between the earth and the sun varies through the yearvaries through the year

We are closest at We are closest at perihelionperihelion; we are ; we are furthest apart at furthest apart at aphelionaphelion ( (aapart/part/aaphelion)phelion)

AphelionAphelion (furthest from sun) occurs (furthest from sun) occurs on on July 4July 4thth (152 million km) and (152 million km) and

PerihelionPerihelion (closest to sun) occurs on (closest to sun) occurs on January 3January 3rdrd (147 million km) (147 million km)

Closest in January???

Then why is it coldest???

What causes the seasons??What causes the seasons??The tilt of the earth on its axis!!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 23Equinox: means “equal nights”; length of day and night are equal all over the worldThe 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 LatitudeDaylight Change by Latitude

Monthly Variation in Daylight in Three Cities

0

5

10

15

20

25

30

Month

Da

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gh

t H

ou

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Narragansett Tromsö Mexico City

Can you detect the times of equinox?

Can you detect the summer and winter solstices?

What is an eclipse?What is an eclipse?An eclipse occurs when one planetary body An eclipse occurs when one planetary body

passes through the shadow of another.passes through the shadow of another.Eclipses are named depending on which Eclipses are named depending on which

body is eclipsed, or blockedbody is eclipsed, or blocked

Solar Eclipse: Solar Eclipse: (the sun is eclipsed or blocked)(the sun is eclipsed or blocked)

TThe moon is between the sun and earth - the he moon is between the sun and earth - the shadow of the moon falls on the earthshadow 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 EclipseLunar Eclipse: : (the moon is eclipsed, or blocked) (the moon is eclipsed, or blocked)

The earth is between the moon and the sun - the The earth is between the moon and the sun - the earth’s shadow falls on the lighted half of the earth’s shadow falls on the lighted half of the

moonmoon

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 PhasesMoon Phases

Phases of the Moon: Phases of the Moon: As Moon revolves around the Earth, As Moon revolves around the Earth,

different parts of the lighted side of the different parts of the lighted side of the moon face Earthmoon face Earth

New moon: moon is between sun and New moon: moon is between sun and earth – no visible moonearth – no visible moon

Full moon: earth is between sun and moon Full moon: earth is between sun and moon – fully visible moon– fully visible moon

Crescent moon: < half of moon visibleCrescent moon: < half of moon visible Gibbous moon: > half of moon visibleGibbous moon: > half of moon visible Waxing: visible part is increasing Waxing: visible part is increasing Waning: visible part is decreasingWaning: visible part is decreasing

Moon PhasesMoon Phases

New moon → waxing crescent → first quarter → waxing gibbous → full moon → waning gibbous → last quarter → waning crescent → back to new moon

Phases of the MoonPhases of the Moon

Tides: daily changes in the level of Tides: daily changes in the level of the ocean surfacethe ocean surface

Caused by Caused by the the gravitational gravitational pull of the pull of the moon (and moon (and the sun) on the sun) on the Earth’s the Earth’s waterswaters

Both Sun and Moon Both Sun and Moon cause the tidescause the tides

Spring tides: Spring tides: S-E-MS-E-M

Neap Tides: Neap Tides: S-ES-E M 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 TidesNeap and Spring Tides

Tides – 3 tidal patternsTides – 3 tidal patterns diurnal tidesdiurnal tides: one high, : one high,

one low every 24 one low every 24 hours hours

semidiurnalsemidiurnal tides: two tides: two equal high, two equal equal high, two equal low every 24 hourslow every 24 hours

semidiurnal mixed semidiurnal mixed tidestides: 2 highs of : 2 highs of different heights, 2 different heights, 2 lows of different lows of different heights every 24 hoursheights every 24 hours

Tidal rangeTidal range: : difference in height difference in height between the between the highest high and highest high and the lowest low tidethe lowest low tide

Tidal period: time Tidal period: time between one high between one high tide and next high tide and next high tidetide

Tidal day: time Tidal day: time between 1between 1stst high high tide and last high tide and last high tide in 24 hourstide in 24 hours

What kind of tides in Eastport, ME?What kind of tides in Eastport, ME?Look at the daily tidal range!Look at the daily tidal range!

What kind of tides in Grand Isle, LA?What kind of tides in Grand Isle, LA?Look at the daily tidal rangeLook at the daily tidal range

What kind of tides in What kind of tides in Crescent City, CA?Crescent City, CA?

What kind of tides in Newport, RI ?What kind of tides in Newport, RI ?Look at the daily tidal rangeLook at the daily tidal range