The Four Seasons

Is the changing seasons caused by the change in the distance between the Sun and the Earth?

No. If it is, then – The northern and southern hemisphere should have

the same season, not opposite season like we have.

– We should experience real seasonal changes in Hawaii also.

– Northern and Southern Hemispheres have the opposite seasons as a result of the tilt of the Earth’s axis

Factors Affecting Earth’s Temperature

Distance to heat source

Illumination Angle

Noon, Summer Solstice

Sunrise/Sunset,Winter Solstice

Does NOT produce the Earth’s seasons!!!

Click on the Sun to start animation

Solstices and Equinoxes• Equinox: An equinox is one of two opposite points

on the celestial sphere where the celestial equator and ecliptic intersect.

• Solstice: A solstice is either of the two times of the year when the sun is at its greatest distance from the equator.

– Spring Equinox ~ March 21– Summer Solstice ~ June 21– Fall Equinox ~ September 22– Winter Solstice ~ December 21

• The dates of the equinoxes and solstices are only approximate dates. – The actual length of a year is about 365 ¼ days (365

days, 5 hours, 49 minutes), not exactly 365 days. We have to add an extra day to a year every four years to keep the seasons synchronized with the seasons (leap year). Over a longer period of time, we need to skip a leap year to compensate the extra minutes we add in every leap year to keep the calendar in sync.

Celestial Equator

Ecliptic Plane

Spring Equinox

Fall Equinox

Winter Solstice

Summer Solstice

The Reasons for the Seasons on Earth

• The seasons on Earth are caused by the 23.5º tilt of its rotation axis, and its revolution around the Sun.

• Although the distance of the Earth to the Sun changes slightly as it orbits around the Sun, its effect is not big enough to cause the four seasons.

• The seasons for other planets may be due to the changing distance to the Sun, not the tilt of their rotation axis…For example, the orbit of Pluto has a very large eccentricity.

The Effects of Precession

What changes do you expect in these things (or phenomena)13,000 years later when the Earth’s rotation axis is pointedtoward Vega?

A. World Atlas (map of Earth)? No change.B. Star chart (map of sky)?

- Location of celestial north would be different…(Rotational axis will point at Vega)

C. Seasons?− We will still have four seasons, since the tilt of the rotation axis of Ea

rth with respect to the ecliptic plane is not changed. But the time of the season would be changed. Summer would be in January, and winter would be in July.

D. Constellations?− The patterns of stars in the sky would not change much…but their

position with respect to celestial north would be different).

Eclipses and Lunar Phases

http://solarsystem.nasa.gov/multimedia/gallery/Earth_Moon.jpg

By the Lunar and Planetary Institute

Preliminary TopicsTo understand the reason for phases,

first understand:

• The Moon orbits the Earth

• The Moon orbit at an angle with respect to the Earth’s orbit around the Sun

• The Moon doesn’t shine on its own; it reflects sunlight

• The Sun is 400x larger than the Moon, but it is also 400x further away from the Earth

• The Moon has approximately 1/4 Earth's diameter, 1/50 Earth's volume, and 1/80 Earth's mass.

• The Moon reflects visible light about 1/3 as well as Earth, and because of its much smaller size, has a visual brightness less than 1/40 that of Earth

Ecliptic planeMoon’s orbital plane

Motions of the Sun-Moon-Earth System1. Earth orbits the Sun every 365 ¼ days2. Moon orbits Earth every 29 ½ days

This is what the orbit of the Moon looks like…

3. The orbit of the Moon around Earth is tilted about 18.5 degrees with repect to the ecliptic plane

4. Moon’s orbit around Earth precesses with a period of 18.5 years

starchild.gsfc.nasa.gov/docs/StarChild/questions/phases.html

The Moon

The moon revolves around the earth with a period of about 29 ½ days ~ one month!

Facts about the Moon…• We see the moon changes its

appearances and position in the sky with approximately 30-day cycle.

• Unlike the stars, Moon can also be seen during the day.

• We also see that the Moon is always involved in eclipses, and

• Its position seems to be well correlated with the tide of the ocean.

• Understanding the motion of the Moon with respect to the Sun and the Earth will explain these phenomena.

The Face and Phases of the Moon

• The phase (how much and which side of the Moon is illuminated) of the Moon changes with about one month period…

• The face (the features on the surface of the Moon) does not change at all…

Phases--Causes

• The Sun shines on the Moon. – When the sunlight reflects off the Moon’s far

side, we call it a New Moon– When the sunlight reflects off on the Moon’s

near side, we call it a Full Moon– Between New and Full, we see parts of the

daytime side of the Moon.

New (couple days)

Waxing Crescent (several days)

1st Quarter

Waxing Gibbous (several days)

Full

Waning Gibbous (several days)

3rd Quarter

Waning Crescent (several days)

New

Phases: Observing and Identifying

http://www.lpi.usra.edu/education/skytellers/moon_phases/about.shtml

The Phase of the Moon

The phase of the Moon depends on the relative position between the Sun, the Earth, and the Moon…

• The changing phase of the Moon…• The un-changing face of the Moon…• Eclipses

– Lunar Eclipses– Solar Eclipses

Preliminary TopicsTo understand the reason for phases,

first understand:

• The Moon orbits the Earth

• The Moon orbit at an angle with respect to the Earth’s orbit around the Sun

• The Moon doesn’t shine on its own; it reflects sunlight

• The Sun is 400x larger than the Moon, but it is also 400x further away from the Earth

• The Moon has approximately 1/4 Earth's diameter, 1/50 Earth's volume, and 1/80 Earth's mass.

• The Moon reflects visible light about 1/3 as well as Earth, and because of its much smaller size, has a visual brightness less than 1/40 that of Earth

Ecliptic planeMoon’s orbital plane

Distance Between the Moon and Earth

• Like the Earth’s orbit around the Sun, the orbit of the Moon around the Earth is not a perfect circle. – Eccentricity of Earth’s orbit is

~ 0.016

• The distance between the Moon and Earth change. This is why we have total and ring eclipses– Eccentricity of Lunar orbit is

~ 0.05

Solar Eclipses: A Coincidence?

• What if the Moon is closer (say twice as close) to the Earth?

• What if the Moon is farther (say twice as far) away from the Earth?

• What if the Moon is two-time bigger? • What if the Moon is two-time smaller?

Solar and Lunar EclipsesEclipse:

The total or partial obscuring of one celestial body by another…

The obscuration can be either• One celestial body blocking

the view to the other:– Solar eclipse---Moon

blocking Earth’s view to the Sun…

• One celestial body is in the shadow of another:– Lunar eclipse---Moon is in

the shadow of the Earth…

Lunar eclipse image from http://www.mreclipse.com

What Causes Eclipse?• The Earth and Moon cast shadows.• When either passes through the other’s shadow, we have an eclipse.• Because the Sun is an extended bright object, there are two different

regions of the shadow:– Penumbra is partially illuminated– Umbra is completely dark

Lunar Eclipses

• The Sun and Moon occasionally line up so that we have an eclipse.

– These eclipses happen every year

– To see a solar eclipse, you need to be on a particular part of the Earth

When the Earth’s shadow covers the Moon, we have a lunar eclipse

Three types of Lunar Eclipses

• Penumbral lunar eclipse—the Moon only passes through the penumbra of Earth’s shadow

• Partial lunar eclipse—part of the Moon passes through the umbra of Earth’s shadow

• Total lunar eclipse—the entire Moon passes through the umbra of Earth’s shadow

• Who on Earth will be able to see a lunar eclipse?Anyone who can see the Moon (anyone who is

on the nighttime side of the Earth during the eclipse)

Lunar eclipses

• Lunar eclipses happens when the Moon passes through the shadow of the Earth– Everybody on the night side

of Earth can see the lunar eclipse.

• Lunar eclipses can be partial, penumbral, or total.

• Lunar eclipses can occur only at full moon.

Images from Fred Espenakhttp://www.mreclipse.com/LEphoto/LEgallery1/LEgallery1.html

Why is the Moon red during an eclipse?

• The Earth’s atmosphere filters some sunlight and allows it to reach the Moon’s surface

• The blue light is removed—scattered down to make a blue sky over those in daytime

• Remaining light is red or orange• Some of this remaining light is bent or

refracted so that a small fraction of it reaches the Moon

• Exact appearance depends on dust and clouds in the Earth’s atmosphere

Solar Eclipses

The solar corona is revealed during a total solar eclipse• The corona is about one

millionth times fainter than the disk of the Sun.

• Similar to observing stars next to the Sun, the light from the disk must be blocked (by the moon, or by special occulter in the telescope) before we can see the solar corona.

Solar eclipses

• Solar eclipses occur when the shadow of the Moon falls on the surface of Earth– Only people in the shadow

can see the eclipse.

• Solar eclipses can be partial, annular, or total.

• Solar eclipses can occur only at new moon.

Click on the image to start animation

Solar Eclipse ForecastSolar eclipses from 2004 to 2030

Knowing the orbit of the Earth and the Moon, we can now calculate the time and path of solar eclipses with great accuracy.

Back to Eclipse Path

Eclipse Path

When total or ring solar eclipse happens…• The diameter of the umbra of lunar shadow is no more than

270 km…you can see the total eclipse only if you are in a very narrow and long eclipse path.– In the 1990 total eclipse that passed through the big island of Hawaii,

people on Maui and Oahu cannot see the total eclipse!• The diameter of the penumbra of the lunar shadow is about

7000 km (Earth’s diameter is about 13,000 km). So, the region that partial eclipse can be seen is quite large…and people on Maui and Oahu could see partial eclipse!

Solar Eclipses• When the Moon’s shadow covers part of the

Earth• Only happens at New Moon• Three types: Annular, Partial, and Total

Total Solar EclipseWhat’s the difference between a total and ring eclipse?The distance between the Earth and the Sun.

You see the whole Sun outside of the shadow

You see the partial Sun if you are in the penumbra

You don’t see the Sun at all if you are in the umbra

Moon

Surface of the Earth

Sun

Click on the Sun to start animation

Total Solar Eclipse• Observers in the “umbra” shadow see a total eclipse (safe to view the

Sun); can see the corona• Those in “penumbra” see a partial eclipse—not safe to look directly at

Sun• Only lasts a few minutes• Path of Totality about 10,000 miles long, only 100 miles wide

Ring (Annular) Eclipse• Ring eclipse happens when the Moon does not

completely block the Sun, like partial eclipses…

You see the whole Sun if you are outside of the shadow

You see the partial Sun if you are in the penumbra

• The umbra is NOT completely dark!

• You see a ring if you are in here

Moon

Surface of the Earth

Sun

Click on the Sun to start animation

Annular Solar Eclipse• When the Moon is too far to completely cover the Sun—the

umbra doesn’t reach the Earth• Sun appears as a donut around the Moon

Eclipse Path

When total or ring solar eclipse happens…• The diameter of the umbra of lunar shadow is no more than

270 km…you can see the total eclipse only if you are in a very narrow and long eclipse path.– In the 1990 total eclipse that passed through the big island of Hawaii,

people on Maui and Oahu cannot see the total eclipse!• The diameter of the penumbra of the lunar shadow is about

7000 km (Earth’s diameter is about 13,000 km). So, the region that partial eclipse can be seen is quite large…and people on Maui and Oahu could see partial eclipse!

How Often Does Eclipse Occur?• Do we have one solar and one lunar eclipse every six month,

when the nodes line-up with the Earth-Moon line?– NO! Because of the precession of the lunar orbit!

Nodes: the two points when the Moon crosses the ecliptic plane• Only when the

nodes are located right between the Earth-Sun line can solar eclipses occur.

The Precession of Lunar Orbit

• The orbital motion of the Moon around Earth slowly precesses with an 18.6 year cycle as the Earth orbit the Sun…

How Often Does Eclipse Occur?

• The combination of these motions of the Moon1. 29 ½ day orbital motion around Earth,2. 5º tilt of the orbit w.r.t. the ecliptic, and3. Precession of the lunar orbit w.r.t. the ecliptic,

• Solar Eclipse occurs with a period of about 18 years and 11-1/3 days Soras cycle

• But these eclipses don’t happen at exactly the same location over the Soras cycle…

Eclipses: Summary• The parties involved: Sun, Moon, and Earth• Motion of the Moon around Earth:

1. 29 ½ day revolution of the Moon around the Sun2. Tilt (~5º) of the lunar orbit (around the Earth) w.r.t. the Ecliptic plane (Earth’s

orbital plane around the Sun)3. The precession of the lunar orbit w.r.t. Earth-Sun direction

• Solar eclipse happens when the Moon is between the Earth and the Sun.• The size and distance of the Moon need to be just right for us to see total

eclipse.– The changing distance between the Earth and the Moon explains the

occurrences of the total and ring solar eclipses. – The changing distance between the Earth and the Sun, and the Earth and the

Moon explains the difference in the duration of the solar eclipses. • Lunar eclipse happens when Earth is between the Moon and the Sun.

Photo of a Total Eclipse

http://sunearthday.nasa.gov/2006/multimedia/gal_008.php

Photos of an Annular Eclipse

http://sunearthday.nasa.gov/2006/multimedia/gal_010.php; photos taken by Fred Espenak

Solar Eclipse ForecastSolar eclipses from 2004 to 2030

Knowing the orbit of the Earth and the Moon, we can now calculate the time and path of solar eclipses with great accuracy.

Back to Eclipse Path

Upcoming Solar EclipsesInsert upcoming Solar Eclipse dates• Next Total Solar Eclipse in USA—August 21,

2017