1.3THE CHANGEABLE

MOON

Phases of the Moon

The lunar phases are caused by the changes in how much of the illuminated (sunlit) side of the moon faces Earth. Half the moon is always

illuminated!

The moon produces none of its own light.

Phases of the Moon

1st Phase NEW MOON None of the moon appears illuminated as

viewed from Earth.

Waxing – phases during which the lit portion of the moon increases from the RIGHT.

Waning – phases during which the lit portion of the moon decreases and only the LEFT hand side remains illuminated.

The Sky At Sunset

The Sky At Sunrise

Lunar Motions The cycle of the moon through all 8 phases

takes 29.5 days. This is one complete revolution with respect to the

Sun and is called a synodic month.

The true period of revolution for the moon is actually 27.3 days. This revolution is known as a sidereal month, and is

with respect to distant stars. The reason it takes longer to go through the phases is

because as the moon is revolving around Earth, the Earth-moon system is also revolving around the Sun.

Synodic vs. Sidereal Month

Lunar Motions

The moon is not only revolving around the Earth, but also rotating upon its own axis. One complete rotation on its axis

takes 27.3 days. You may recall this number is the

sidereal month time.

Therefore, the moon revolves around the Earth at the same rate it rotates on its axis. This is called synchronous rotation.

Lunar Eclipses When the Earth is positioned between the

Sun and the moon, casting its shadow on the moon, a lunar eclipse occurs.

A lunar eclipse occurs during the full moon phase, as viewed from the Earth.

The zone of full shadow is known as the umbra.

The zone of partial shadow is known as the penumbra.

A B

Solar Eclipse

Lunar Eclipse

Solar Eclipses When the moon

moves in a line directly between Earth and the Sun, casting a shadow on Earth, it is known as a solar eclipse.

A solar eclipse occurs during the new moon phase, as viewed from the Earth.

Solar Eclipses

Total Solar Eclipse During a total solar eclipse, the

moon completely blocks out the solar disk of the Sun for up to seven minutes. Temperature sharply decreases a

few degrees during this time.

Visible only to people within the moon’s umbra. A partial eclipse is seen by those in moon’s penumbra.

Total solar eclipses are very rare at any location. Next one visible from the United States 8-21-17

Total Solar Eclipse

Total Lunar Eclipse During a total lunar eclipse, the

moon is completely within Earth’s umbra, but is still visible as a reddish-orange disk.

• This is due to Earth’s atmosphere scattering sunlight and the longer red wavelengths reflect off the moon.

• Visible to anyone on the side of the Earth facing the moon.

• Total lunar eclipse can last up to 1 hour and 40 minutes!

Angular Diameter

Solar eclipses are spectacular because Earth’s moon has nearly the same angular diameter as the Sun, so it covers it almost exactly.

The angular diameter is the angle formed by lines extending toward you from opposite edges of the object and meeting at your eye.

The linear diameter is simply the distance between an object’s opposite sides. Example: ordering a pizza (16 inch)

Angular Diameter

To find angular diameter, you need to use the following formula:

Your answer will always be in arc seconds since 206,265 is the number of arc seconds in 1 radian (basis for the formula).

Example: Find the angular diameter of the moon as viewed from

Earth. Hint: the moon has a linear diameter of 3476 km and a distance from Earth of about 384,000 km.

Small-Angle FormulaAngular Diameter = Linear Diameter

206,265” distance

1870” (arc-seconds)

Angular Diameter

≈ 0.5o = 31’ = 1870”

Angular Diameter Example

Planet Freeport is 6.5 x 107 km. away from it’s star, Yellowjacket, which experience solar prominences extending up to 4.0 arc minutes (’) from its disk. How far is this in kilometers?

Hint: The question is asking for the linear diameter.

Yellowjacket 76,000 km.

Sun

Earth

Moon

(Eccentricities greatly exaggerated!)

Perihelion = position closest to the Sun

Aphelion = position furthest away from the Sun

Perigee = position closest to the Earth

Apogee = position furthest away from the Earth

Apogee vs. Perigee

Apogee vs. Perigee

The orbit of the moon is slightly elliptical and its distance from Earth varies. When the moon is at apogee, its farthest point from Earth, its

angular diameter is about 6% smaller than average. When the moon is at perigee, its closest point to Earth, its

angular diameter is 6% larger than average.

Due to Earth’s slightly elliptical orbit, it is closest to the Sun in January (yes, in winter) and the Sun looks about 1.7% larger in angular diameter.

When the Earth is at its farthest point from the Sun, it looks 1.7% smaller in angular diameter.

Changes in Angular Diameter

(Perigee)

(Apogee)

(Perihelion)

(Aphelion)

Annular Solar Eclipse

If the moon crosses in front of the Sun when the moon’s disk is smaller in angular diameter than the Sun’s, it produces what is known as an annular solar eclipse. This is a solar eclipse in which a

ring, or annulus, of light is visible around the disk of the moon.

An annular eclipse swept across the United States on May 10, 1994.

Annular Solar Eclipse

Features of Solar Eclipses The totally eclipsed Sun is a spectacular sight!

With the moon covering the bright surface of the Sun, known as the photosphere, you can see the Sun’s faint outer atmosphere, the corona, glowing with a pale, white light.

The thin layer of bright gas just above the photosphere is known as the chromosphere.

Eruptions on the solar surface, called prominences, glow with a clear, pink color due to the high temperature of the gases involved.

Just as totality begins or ends, a small part of the photosphere can peek through at the edge of the lunar disk. Forms a silvery ring of light with a brilliant bright spot of

photosphere gleaming like a diamond, known as the diamond-ring effect. Not visible during every solar eclipse.

Chromosphere and Corona

Prominences

The Diamond-Ring Effect

Predicting Eclipses

We would have a lunar eclipse with every full moon and a solar eclipse with every new moon if the orbit of the moon were on a flat plane. However, we know it is tilted about 5°.

Each month, the moon crosses the ecliptic at 2 points called nodes.

The node it crosses as it moves southward is known as the descending node and the one is crosses moving northward it the ascending node.

Predicting Eclipses2 conditions necessary for an eclipse to occur:

1. The Sun must be near a node.

2. The moon must be crossing the same node (solar eclipse – new moon) or the opposite node (lunar eclipse – full moon).

Predicting Eclipses

The Sun’s gravitational influence causes the nodes to slightly move (about 19.4° westward each year from our view on Earth). Result the Sun does not need a full year to go from a

specific node all the way around the ecliptic and back to that same node.

The Sun will cross the same node after only 346.6 days, what is known as an eclipse year. This means the eclipse seasons begin about 19 days

earlier every year.

The Saros Cycle Eclipses follow certain patterns and cycles, the most important

being the Saros Cycle. After 1 Saros Cycle of 18 years, 11 days, 8 hours, the pattern of eclipses

repeats same Sun-moon-Earth geometry. In fact, Saros comes from the Greek word meaning “repetition”.

1 Saros = 6585.321 days, which = 223 lunar months.

Although the Saros repeats almost exactly, it is not visible from the same place on Earth. The Saros Cycle is 1/3 of a day longer than 18 years, 11 days.

When the eclipse happens again, the Earth will have rotated one-third of a turn farther east, and the eclipse will occur one-third of the way westward around Earth.

Therefore, after 3 Saros Cycles (54 years, 1 month) the same eclipse occurs in the same geographic region of Earth.