Astronomy Lecture 05/06

This motion of the Sun on the sky traces out a path on the celestial sphere known as the

Ecliptic

Ecliptic

The apparent path of the Sun, relative to the stars on the celestial sphere, over the course of a year

Zodiac

The 12 constellations through which the sun moves as it follows the path on the ecliptic

What angle does the ecliptic from on the celestial sphere?

23\.5

What causes seasons?

Depend on the orbit (revolution) of the earth around the sun and how **the tilt of the Earth’s axis** affects the directness of sunlight and length of days

What is Summer Solstice?

Sunlight falls more directly on the Northern Hemisphere, making solar energy more concentrated and making the Sun’s path longer and higher through the sky

What is Winter Solstice?

Sunlight falls more directly on the Southern Hemisphere than the Northern hemisphere

Spring/Fall Equinox

The sun shines equally on both hemispheres

When is Summer solcitce?

June 21

When is Winter solstice?

December 21

When is Spring Equinox?

March 21

When is Fall Equinox?

September 22

What does the Spring equinox mark?

The transition from the Southern Hemisphere being tipped towards the sun to the Northern Hemisphere being tipped towards the sun

What does the Fall equinox mark?

Marks the transition from the Northern Hemisphere being tipped towards the sun to the Southern Hemisphere being tipped towards the sun

On equinoxes, are the days equal length?

Yes

Crossing points

Mark the Spring Equinox and Fall Equinox

Why doesn’t distance matter?

Only small variation of Earth-Sun distance about 3%

This small variation overwhelmed by effects of axis tilt

Because of its relationship to changing seasons, ancient cultures attached great significance to the ecliptic

Marks the center line of the Zodiac, a band of the sky divided into 12 parts of 30 degrees. (No longer of special importance in astronomy)

Evening star

Any planet in the sunset sky

Morning star

Any planet in the dawn sky

Does Venus seasonally switch from morning to evening star?

Yes

When does the axis of the Earth tilt?

Every 26,000 years

How close is the Moon to the Earth?

380,000 km

When does the Moon orbit the Earth?

Every 27.32 days

Where does the Moon set and rise?

Rise in the east and set in the west every night

What causes the phases of the Moon?

The moon reflects light from the sun

Half of the Moon is illuminated by the Sun and half is dark

We see a changing amount of the bright and dark faces as the Moon orbits

New Moon

Occurs when the Moon lies in the **same direction** as the sun

Full Moon

Occurs when the Moon is **opposite** the sun

Synchronous rotation

The Moon rotates exactly once with each orbit (why only one face is visible from the Earth)

What causes eclipses?

The Earth and Moon cast shadows

When either passes through the other’s shadow

Why don’t we have an eclipse at every new and full moon?

The Moon’s orbit is tilted 5 degrees to ecliptic plane

So we have about 2 **eclipse seasons** each year, with a lunar eclipse at full moon and solar eclipse at new motion

How much of the moon’s surface is at all times illuminated by the sun?

Half towards the sun, the other half is dark

What is a Lunar Eclipse?

* **Definition:** A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting a shadow on the Moon.
* **Types:** There are three types of lunar eclipses: total, partial, and penumbral.
* **Appearance:** During a total lunar eclipse, the Moon appears reddish-orange due to the Earth's atmosphere bending sunlight towards it.
* **Frequency:** Lunar eclipses occur about twice a year, but may not be visible from all locations.
* Can only occur at **full moon** when the moon is on the other side of the Earth

What causes eclipses?

When the Moon passes between the Sun and Earth, casting the Moon’s shadow on Earth

Solar Eclipse

can only occur at new moon, when the moon is in between the sun and the earth

annular eclipse

when the moon crosses in front of the sun it is too small to fully cover the sun

2 Conditions to have an eclipse

1. must be a full moon (for a lunar eclipse) or new moon (for a solar eclipse)
2. the Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes)

What’s the phase of the moon needed for a lunar eclipse?

Full Moon

The sun, earth, and moon all have to be aligned

What’s the difference between a lunar and solar eclipse?

* A lunar eclipse occurs when the Earth passes between the sun and the moon, and the Earth's shadow falls on the moon
* A solar eclipse occurs when the moon passes between the sun and the Earth, blocking the sun's light from reaching the Earth