nats 1311 - from the cosmos to earth our first exam will be next tuesday, september 23 at the...

NATS 1311 - From the Cosmos to Earth

Our first exam will be next Tuesday, September 23 at the regular class time. We will have a review Thursday (Sept. 18) - at a time to be determined in class today - in the room next to where the regular lab is held. The exams will cover everything that we have covered in class up to the end of class Thursday.


Eclipses

The Earth & Moon cast shadows.

When either passes through the other’s shadow, we have an eclipse

Solar eclipse - when Earth passes through Moon’s shadow - occurs at new Moon

Lunar eclipse - When Moon passes through Earth’s shadow - occurs at full Moon


Causes of Eclipses Animation


Why don’t we have eclipses every new and full Moon?

Eclipses Twice A Month? Animation


Moon’s Orbital Tilt Animation


Moon’s orbit tilted 5° to ecliptic plane

- Crosses ecliptic plane only at the two nodes

- Eclipse possible only when full/new occur near nodes


Imagine ecliptic plane as surface of pond - points at which Moon’s orbit crosses the surface are nodes. Eclipses only occur at nodes when the Moon is new or full.


The shadow cast by an object in sunlight. Two distinct regions - sunlight is fully blocked in the umbra and partially blocked in the penumbra. Umbral shadow totally dark, penumbral shadow only slightly darker than no shadow.

Shadows

NATS 1311 - From the Cosmos to Earth Types of Lunar Eclipses

TotalMoon in Earth’s umbra - maximum duration in total eclipse region - 140 minutes

PartialMoon partly in Earth’s umbra

PenumbraMoon in Earth’s penumbra - virtually undetectable


As viewed from Earth, Earth's shadow can be imagined as two concentric circles.

- type of lunar eclipse is defined by the path taken by the Moon as it passes through Earth's shadow.


Generally two lunar eclipses per year, separated by 6 synodic months (177 days)

- one on ascending node and one on descending node

Descending node Ascending node


Lunar Eclipses Animation


Evolution of Total Lunar Eclipse Animation


Lunar eclipse sequence

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Total Lunar Eclipse

The reddish color is because the Earth’s atmosphere bends some ofthe red light from the Sun around the Earth to the Moon.


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Total Lunar Eclipses Present - 2015

December 21, 2010

June 15, 2011

December 10, 2011

April 15, 2014

October 8, 2014

April 4, 2015

September 28, 2015

There are typically 2 partial or total lunar eclipses per year.

NATS 1311 - From the Cosmos to Earth Solar Eclipses

We are fortunate - the Sun appears approximately as large in the sky (same angular diameter ~ 0.50) as the Moon.

When the Moon passes in front of the Sun, the Moon can cover the Sun completely, causing a total solar eclipse.


Types of Solar Eclipses

TotalIf the eclipse occurs when the Moon is close to the Earth - umbra covers small area of Earth’s surface (no more than 270 km in diameter) - anyone in this area sees a total eclipse.

PartialAnybody in penumbral shadow (about 7000 km in diameter) will see a partial eclipse.

AnnularIf the eclipse occurs when the Moon is far from the Earth - the umbra does not reach the Earth, only the penumbra. Anyone in this area directly behind the umbra sees an annular eclipse


Evolution of a Partial Solar Eclipse Animation


Evolution of a Total Solar Eclipse Animation


Earth and Moon’s Orbits Are Slightly Elliptical

Sun

Earth

Moon

(Eccentricities greatly exaggerated!)

Perihelion = position closest to the sun

Aphelion = position furthest away

from the sun

Perigee = position closest to Earth

Apogee = position furthest away from Earth


Annular Solar Eclipses

The angular sizes of the moon and the sun vary, depending on their distance from Earth.

When Earth is near perihelion, and the moon is near apogee, we see an annular solar eclipse.

Perigee Apogee Perihelion Aphelion


So eclipses should be easy to predict - every six months at nodes when Moon crosses the ecliptic. However, the Moon’s orbit also precesses making prediction difficult.

Moon’s Orbital Precession Animation


Predicting Eclipses

Solar eclipses are one of the most awe-inspiring events in human history - difficult for ancients to predict because of orbital precession and narrow region of occurrence. First predicted historical solar eclipse date precisely known - May 28, 585 BC. Opposing armies in Turkey massing for battle - eclipse frightened them so much, they put down their weapons and signed a treaty.

Combination of changing dates of eclipse seasons (Lunar orbital precession), 29 1/2-day cycle of lunar phases, and period between lunar perigees --> solar eclipses occur in 18 year, 11 1/3 day cycle - saros cycle.


Eclipses repeat

- after one saros cycle, Moon and nodes of its orbit (and its perigee) return to same place with respect to the Sun

- one saros contains 223 lunar months

- changes location on Earth because of the extra 1/3 day - moves 8 hours of longitude west of the previous eclipse

- repeats in same location after 3 saros cycles

Saros Cycle


Paths of Total Solar Eclipses

Paths of same color represent successive saros cycles.


Partial solar eclipses

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Shadows from partial solar eclipses

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Annular eclipses

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Total solar eclipses

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Total solar eclipsesequences.


Total solar eclipses

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Baily’s beads

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Solar eclipse--the Moon’s shadow seen from space

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Antikythera mechanism

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Einstein’s eclipse

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nats 1311 - from the cosmos to earth our first exam will be next tuesday, september 23 at the...

Documents

earth nats

earth lunar eclipse

earth solar eclipseswe

earths shadow

moons shadow

type of lunar eclipse

total solar eclipse

shadow cast