ECLIPSES Two types! Lunar Eclipse = The moon passes behind ...cmcasey/ast307_fa16/lec05.pdf ·...

E C L I P S E S Two types!

Lunar Eclipse = The moon passes behind the

Earth’s shadow and makes the moon appear red.

Solar Eclipse = The moon passes in front of the sun and casts a shadow

on the Earth.

H O W D O E C L I P S E S W O R K ?

Both the moon & Earth cast shadows in space. The areas of space where the sun is completely invisible are shaped like cones

(if you are farther away, the shadow subtends a smaller angle on the sky).


The shadow region which has complete blockage of sunlight is called the umbra, partial is preumbra.

umbra



umbra

The spaceship can only see PART of the sun from its position in the preumbra.



penumbra

umbra


Solar Eclipses happen when the moon is between the Earth & Sun, i.e. during

“New Moon”


Lunar Eclipses happen when the Earth is between the

moon & Sun, i.e. during “Full Moon”

penumbra

umbra

Lunar eclipses are when sunlight is prevented from hitting the surface of the moon due to the Earth’s shadow.

The reason the surface of the moon appears red is due to sunlight refracted through

the Earth’s atmosphere.Moon passi

ng through Earth

’s shadow

With your group, discuss why solar eclipses happen (why the moon blocks the sun completely).

What do you know about the diameter of the sun?

One useful piece of information: the distance to the sun is 1 A.U. = 1.49 x 108 km (A.U. stands for “astronomical unit”)

(not to scale!!!!)1 A.U.

Annular eclipse When the moon is slightly farther

in its orbit around Earth it’s angular size on the sky is just smaller than 0.5o and the sun appears as a very bright ring.

Image credit: Mikael Svalgaard

What type of phase can the moon have during a lunar eclipse?

A. New Moon B. Full Moon C. Waning Crescent to

Waxing Crescent D. Waxing Gibbous to

Waning Gibbous

What type of phase can the moon have during a solar eclipse?

A. New Moon B. Full Moon C. Waning Crescent to

Waxing Crescent D. Waxing Gibbous to

Waning Gibbous

W H Y D O N ’ T E C L I P S E S H A P P E N A L L T H E T I M E ( I . E . E V E R Y N E W A N D F U L L M O O N ? )

The moon’s orbit is tilted with respect to the Earth’s orbit around the sun, and so “favorable” eclipse conditions only happen approximately twice a year.

Eclipses of both varieties happen roughly around the time of the equinoxes.

Eclipses of both varieties happen roughly around the time of the equinoxes.

Lunar eclipses: Earth’s shadow covers the whole face of the moon (plus some)

Solar eclipses: moon’s shadow only covers small region on Earth.

What we have learned

Lunar eclipses happen when the Earth casts a shadow on the FULL moon

Eclipses happen roughly every six months, with lunar and solar eclipses ~2 weeks apart

Solar eclipses happen when the NEW moon casts a shadow on the Earth, blocking sunlight

Partial eclipses happen when either the Earth or moon are in the penumbra, not the umbra.

S E A S O N S

What causes seasons?

(B) The relative distance between the Earth and the sun.

(D) Changes in the Earth’s tilt within the year.

(C) The Earth’s tilt & concentration of sunlight.

(A) Variation of Sun’s energy output over the course of a year.

What causes seasons?If it were the relative distance between the

Earth and the Sun, then the entire planet would have the same season at the same time.

What causes seasons?If it were the relative distance between the

Earth and the Sun, then the entire planet would have the same season at the same time.

This isn’t true: northern & southern hemispheres

experience winter & summer 6 months apart. Earth’s orbit

almost perfectly circular.

What do I mean by “intensity” or “concentration” of sunlight?

2. Explore how the light coming out of the hole in the box changes when you hold it at different distances

from your grid paper. What do you notice?

1. First you will need to attach the box to your phone’s light (come gather tape if needed), like so:You will need:

* One of these

* paper protractor

* paper ruler

* One piece of graph paper

* cell phone with a bright (concentrated) light

6. Describe how do you think this activity relates to sunlight, the Earth, and seasons.

3. Is your phone giving off the same or different amount of energy when you hold it at different distances from your paper?

4. Based on your answers to 2 & 3, how do you think energy output (of your phone’s camera) relates to how bright light appears on your paper, and distance between grid paper and the light source?

5. Now explore what happens when you change the angle of your grid paper (measure with protractor), but keep it at a fixed distance from the light source. What happens?

Please answer these questions as a group & turn in when finished (1 per group).


Two different concepts to explore: 1. distance from light source 2. angle of incidence of light

#1

#2

#1

distance from light source

#1


the sun (or a lightbulb) outputs energy per unit time.

E/t = Power. In astronomy, we call this “luminosity”, or L.

#1




the brightness of a source as observed at a distance is called “Flux” and is Energy / time /

surface area.

#1




the brightness of a source as observed at a distance is called “Flux” and is Energy / time /

surface area.

Think about: If you are 1m away from a light bulb, how much brighter (or dimmer) is it than if you were standing at 5m? Then…

Draw the relationship between distance from the light bulb (x-axis) and the light’s flux (y-axis).

distance

flux

distance

flux

Flux falls off as:

F / 1

distance2

distance

flux

Flux falls off as:

F / 1

distance2

F =L

4⇡r2

distance

flux

Surface area of a sphere = 4⇡r2

Flux falls off as:

F / 1

distance2

F =L

4⇡r2


Technical term for this is solar irradiance, or flux ~1000 W/m2

What do I mean by “intensity” or “concentration” of sunlight?At high latitude (61o N, Anchorage, AK):

At low latitude (21o N, Honolulu, HI):

dak

dhi

Earth is tilted ~23.5o with respect to plane of solar system; this does not change in a year.

in the northern hemisphere

in the northern hemisphere

Angle of incidence of sunlight is fixed with or without the Earth’s tilt. The Earth’s tilt means that irradiance hits the Earth differently

throughout the year.

Do you think the difference in the distance to the sun between the equator/tropics and

pole/arctic impacts the temperature on Earth?

Not to scale!

1 AU = 1.49 * 108 km

1 Re = 6.4 * 103 km

Do you think the difference in the distance to the sun between the equator/tropics and

pole/arctic impacts the temperature on Earth?

Not to scale!

1 AU = 1.49 * 108 km

1 Re = 6.4 * 103 km

1A.U. ⇡ 1A.U.+ 1Re

Does the sun always rise directly in the East?

Does the sun always rise directly in the East?

No! It actually migrates throughout the seasons, from northeast to southeast…

Fraction of 24 hours spent in daylight.

Fraction of 24 hours spent in night.



35%

Daytime 65%

50%Daytime

50% 65%

Daytime 35%

Summer Solstice June 21 in NH, Dec 21 in SH

Fall/Spring Equinox March 21/Sept 21

Winter Solstice Dec 21 in NH, June 21 in SH



35%

Daytime 65%

50%Daytime

50% 65%

Daytime 35%

Summer Solstice June 21 in NH, Dec 21 in SH

Fall/Spring Equinox March 21/Sept 21

Winter Solstice Dec 21 in NH, June 21 in SH

The relative variation of this fraction depends on latitude: above the arctic circle, you can get days of

100% sunlight in summer and 100% darkness in winter.

W H AT ’ S S P E C I A L A B O U T T H E A R C T I C /A N TA R C T I C C I R C L E S & T R O P I C S ?

A R O U N D T H E E Q U AT O R , T H E R E L AT I V E D AY L I G H T D O E S N ’ T C H A N G E M U C H , B U T T H E M O O N C A N T E L L Y O U T H E S E A S O N …

March

Septem

ber

Ap

ril

May

June

July

Aug

ust

Octo

ber

No

vemb

er

Decem

ber

January

February

Rainy Season

Ngas Culture in Nigeria used ‘tilt’ of moon to tell when the

rains were coming.

S U M M E R AT T H E S O U T H P O L E .

“The land of the midnight sun”

S U M M E R AT T H E S O U T H P O L E .

“The land of the midnight sun”

xSouth Celestial Pole

also zenith at the south pole

W H AT W E H AV E L E A R N E D .

Seasons are caused by the Earth’s tilt, and the way that the intensity of sunlight (or irradiance)

is spread over the surface of the Earth.

Seasons are not caused by being closer/further to the sun within the cycle of a year.

Learned about FLUX:how we measure light from

distant objects

W H AT W E H AV E L E A R N E D .

Seasons are caused by the Earth’s tilt, and the way that the intensity of sunlight (or irradiance)

is spread over the surface of the Earth.

Seasons are not caused by being closer/further to the sun within the cycle of a year.

Learned about FLUX:how we measure light from

distant objectsF =

L

4⇡r2

ECLIPSES Two types! Lunar Eclipse = The moon passes behind ...cmcasey/ast307_fa16/lec05.pdf ·...

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