1. Eclipses
On multiple occasions throughout the year, the Sun, Moon, and Earth will align to create a lunar or solar eclipse
With the use of precise timing and knowledge of the orbits of the Earth and Moon, it is possible to predict these events in advance
Not all eclipses are the same, and their appearance can change depending on the alignment and the apparent size of the Moon

2. Apparent Size of the Moon and Sun
The size of an object on the sky is expressed as an angle
From horizon to horizon is 180o
The size of your outstretched fist is about 10o
Astronomical objects are usually quite small
Remember, 1o is made up of 60 arcminutes
1 arcminute is made up of 60 arcseconds
The apparent size of the Sun is about 32 arcminutes
The apparent size of the Moon is about arcminutes
Remember that the Moon is sometimes closer and sometimes farther away, so this number changes over the course of a month
The fact that the Sun and the Moon are so close in apparent size creates the spectacular eclipses we see here on Earth

3. Solar Eclipse
For a solar eclipse to occur, the Moon must block out the light from the Sun
This means the Moon must be in between the Sun and the Earth
Notice that this position corresponds to a New Moon
Therefore, a solar eclipse must always occur around the time of New Moon

4. The moon passes through these nodes at least twice per year.
Solar Eclipse
If it has to be a New Moon for a solar eclipse, why doesn't one happen every month?
Remember that the path of the Moon through the sky is slightly tilted compared to the path of the Sun
Only when the paths cross are conditions right for an eclipse
The points where the paths of the Moon and the Sun cross are called nodes.
The moon passes through these nodes at least twice per year.

5. Types of Solar Eclipse - Total
When the Moon is closer to us (apogee), its apparent size matches the apparent size of the Sun
If an eclipse occurs close to apogee, the disk of the Moon will completely block out the light from the Sun
When the disk of the Moon is directly over the Sun, a bright halo of light is visible
This is an outer layer of the Sun called the corona (more on this later in the semester)

6. Types of Solar Eclipse - Annular
If the Moon is farther away from us, its apparent size will be smaller
If an eclipse occurs during this time, the disk of the Moon will not be able to fully cover the disk of the Sun
This results in an annular eclipse
Instead of completely blocking out the Sun's light, a thin ring (or annulus) of light is still visible

7. Solar Eclipse The Right Place at the Right Time
Everyone on Earth does not experience an eclipse in the same way
Only a very few will see the Moon's disk pass directly over the Sun (a total eclipse)
Others will just see the Moon's disk graze the Sun (a partial eclipse)
Darkest part of the shadow : Umbra
Lighter part of the shadow: Penumbra

8. Partial Solar Eclipse

10. Lunar Eclipse
For a lunar eclipse to occur, the Moon must pass through the Earth's shadow
This means the Earth must be in between the Sun and the Moon
Notice that this position corresponds to a Full Moon
Therefore, a lunar eclipse must always occur around the time of Full Moon

11. Lunar Eclipse
Like solar eclipses, lunar eclipses only occur if the Moon is passing through a node
The gray circles at right show the Earth's shadow
Darkest part is umbra
Lighter part is penumbra
Depending on the timing, the Moon will pass through different parts of the shadow, or possibly not at all

12. Penumbral Lunar Eclipse
Penumbral eclipse occurs when the Moon passes through the lighter part of the Earth's shadow
These eclipses are barely noticeable and not particularly interesting

13. Partial Lunar Eclipse
Partial lunar eclipse occurs when the Moon passes through the Earth's umbra, but never goes completely into shadow

14. Total Lunar Eclipse
A total lunar eclipse occurs when the Moon passes completely through the Earth's umbra
Notice below how even when the Moon is completely in Earth's shadow, it still appears to have a red glow
The Earth's atmosphere can act like a lenses and refract light back onto the Moon's surface
Depending on the alignment and atmospheric conditions, the color of the Moon can change from orange to red to simply black

15. Upcoming Lunar Eclipses
2006 Sep 07
Partial
01h33m
Europe, Africa, Asia, Aus.
2007 Mar 03
Total
03h42m 01h14m
Americas, Europe, Africa, Asia
2007 Aug 28
03h33m 01h31m
e Asia, Aus., Pacific, Americas
2008 Feb 21
03h26m 00h51m
c Pacific, Americas, Europe, Africa
2008 Aug 16
03h09m
S. America, Europe, Africa, Asia, Aus.
2009 Feb 09
Penumbral -
e Europe, Asia, Aus., Pacific, w N.A.
2009 Jul 07
Aus., Pacific, Americas
2009 Aug 06
Americas, Europe, Africa, w Asia
2009 Dec 31
01h02m
2010 Jun 26
02h44m
e Asia, Aus., Pacific, w Americas
2010 Dec 21
03h29m 01h13m
e Asia, Aus., Pacific, Americas, Europe

16. Other 'Eclipses' in the Solar System
Occasionally, we can see planets interior to our orbit (Mercury and Venus) cross the face of the Sun
These crossings are called transits
Someone on Mars would have the chance to see an Earth transit
Mercury transits the Sun

17. Planet Transits Around Other Stars
Telescopes and cameras have become so sensitive that they can even detect planetary transits around other stars
Many extrasolar planets have been detected by looking for a dip in the light received from a star

18. Eclipses Outside the Solar System
Most stars come in pairs and are called binary stars
Sometime, the alignment of these stars to our line of sight lets us see one star eclipse the other
You will learn a lot more about eclipsing binaries and their importance in ASTR1020