Lec 4

Question 1

why do we see phases of the moon

Answer 1

As the Moon orbits Earth, it returns to the same position
relative to the Sun in our sky (such as along the Earth–Sun
line) about every 29 1
2 days

This time period marks the
cycle of lunar phases, in which the Moon’s appearance in
our sky changes as its position relative to the Sun changes

Question 2

waxing

Answer 2

phases from new to full; increasing

Question 3

waning

Answer 3

phases from full to new are waning; decreasing

Question 4

crescent

Answer 4

The phases just before and after new moon

Question 5

gibbous

Answer 5

phases just before and after full moon

A gibbous moon is essentially the opposite of a crescent
moon—a crescent moon has a small sliver of light while
a gibbous moon has a small sliver of dark. The term gibbous literally means “hump-backed

Question 6

synchronous rotation

Answer 6

Moon rotates on its axis in the same amount of time it takes to orbit Earth, a trait called synchronous rotation

it is a consequence of Earth’s gravity affecting Moon in much the same way that the Moon’s gravity causes tides on Earth

Question 7

lunar eclipse

Answer 7

occurs when Earth comes directly between the Sun and Moon, so that Earth’s shadow
falls on the Moon

Question 8

solar eclipse

Answer 8

occurs when the Moon comes directly between the Sun and Earth, so that the Moon’s shadow falls on Earth

Question 9

why can a lunar eclipse be seen by anyone on the night side of Earth?

Answer 9

Earth is much larger than the Moon, Earth’s shadow can cover the entire Moon during a lunar eclipse

Therefore, a lunar eclipse can be seen by anyone
on the night side of Earth when it occurs. In contrast, the Moon’s shadow can cover only a small portion of Earth at any moment, so you must be located within the narrow pathway through which the shadow moves to see a solar eclipse

That is why we tend to see lunar eclipses more often than solar eclipses, even though both types occur about equally often

Question 10

full shadow

Answer 10

aka umbra

sunlight fully blocked

Question 11

partial shadow

Answer 11

aka penumbra

light from only part of sun is blocked

Question 12

total lunar eclipse

Answer 12

If the Sun, Earth, and Moon are
nearly perfectly aligned, the Moon passes through Earth’s
full shadow and we see a total lunar eclipse

Question 13

partial lunar eclipse

Answer 13

If the alignment is somewhat less perfect, only part of the full moon
passes through the full shadow (with the rest in the partial
shadow) and we see a partial lunar eclipse

Question 14

penumbral lunar eclipse

Answer 14

If the Moon
passes only through Earth’s partial shadow (penumbra),
we see a penumbral lunar eclipse.

Question 15

totality

Answer 15

Totality begins when the
Moon is entirely engulfed in the full shadow and typically
lasts about an hour, after which we see the shadow gradually move off the Moon

Question 16

why does the moon become dark and eerily red during totality?

Answer 16

The Moon becomes dark and
eerily red during totality, for reasons you can understand by considering the view of an observer on the eclipsed Moon.

This observer would see Earth’s night side surrounded by the reddish glow of all the sunrises and sunsets occurring on Earth at that moment, which means that this reddish light illuminates the Moon during totality

Question 16

total solar eclipse

Answer 16

If a solar eclipse occurs when the Moon is in a part of its orbit where it is relatively CLOSE to Earth, the Moon’s full shadow can cover a
small area of Earth’s surface (up to about 270 kilometers
in diameter). Within this area you will see a total solar
eclipse.

Question 16

annular eclipse

Answer 16

If the eclipse occurs when the Moon is in a PART
of its orbit that puts it farther from Earth, the full shadow
may not reach Earth’s surface, leading to an annular
eclipse—a ring of sunlight surrounding the Moon—in
the small region of Earth directly behind the full shadow

Question 16

partial solar eclipse

Answer 16

when only part of the Sun is blocked from view

(Some
solar eclipses are only partial, meaning that no locations
on Earth see a total or annular eclipse, because the full
shadow passes above or below our planet)

Question 16

how does a total solar eclipse start?

Answer 16

It begins when the disk of the Moon first appears to touch the Sun.

Over the next hour or so, the Moon appears to take a larger
and larger “bite” out of the Sun.

As totality approaches, the
sky darkens and temperatures fall.

During the few minutes of totality, the Moon completely blocks the visible disk of the Sun, allowing the faint corona to be seen

The surrounding sky takes on a twilight glow, and planets and bright stars become visible in the daytime

As totality ends, the Sun slowly emerges from
behind the Moon over the next couple of hours

Question 17

eclipse seasons

Answer 17

Eclipses can occur
only during these periods, called eclipse seasons, which each last about 5 weeks (on average)

In other words, eclipses can occur only when:
1. the phase of the Moon is full (for a lunar eclipse) or
new (for a solar eclipse) and
2. the new or full moon occurs when the Moon is very close
to a node, which means it is during an eclipse season

Question 18

because of the duration of an eclipse season, what events occur?

Answer 18

Because an eclipse season lasts a few days longer than
a cycle of phases, there is always a lunar eclipse (at full
moon) and a solar eclipse (at new moon) during each
eclipse season. (A second lunar or solar eclipse can occasionally also occur during a single eclipse season.)

Question 19

saros cycle

Answer 19

The combination of the changing dates of eclipse seasons and the 29 -day 1
2 cycle of lunar phases makes eclipses recur in a cycle of about 18 years, 111
3 days, called the saros
cycle

Astronomers in many ancient cultures identified the
saros cycle and used it to make eclipse predictions.

For example, in the Middle East the Babylonians achieved remarkable success at predicting eclipses more than 2500 years ago, and the Maya achieved similar success in Central America; in fact, the Maya calendar includes a cycle (the sacred round) of 260 days—almost exactly 11
2 times the 173.32 days between successive eclipse seasons.

Question 20

what other info does the saros cycle give?

Answer 20

However, while the saros cycle allows you to predict
when an eclipse will occur, the approximately 1
3 day in the
cycle length means that the locations where an eclipse will
be visible shift about 1
3 of the way around the world with
each cycle

Question 21

moon phases

Answer 21

The sun illuminates one side of the Moon The other side is in shadow The phase we see depends on where the moon is compared to the sun Phases repeat each time the moon orbits the earth once

Question 22

solar eclipse

Answer 22

When the moon blocks light from the sun Happens when the new moon crosses the ecliptic plane Because the moon’s orbit is tipped relative to ecliptic plane, this happens rarely It only casts a shadow on a small part of the Earth

Question 23

lunar eclipse

Answer 23

When the moon enters the Earth’s shadow Happens when a full moon crosses the ecliptic plane Can be seen from anywhere on earth More common than solar eclipse (earth is larger than moon, so has larger shadow)

Question 24

annular eclipse

Answer 24

Annular eclipse → when moon doesn’t fully cover the sun (caused by moons alignment which isn’t always covering the sun)

Question 25

explain what causes the phases of the moon as seen from Earth

Answer 25

The phases of the Moon are caused by the changing relative positions of the Moon, Earth, and Sun. Half of the Moon is always illuminated by sunlight, just like half of a ball lit by a flashlight. However, from Earth, we see different amounts of the Moon's lit half depending on where the Moon is in its orbit.

Question 26

moon phases

Answer 26

New Moon: The Moon is between Earth and the Sun. The side facing Earth is dark, so the Moon is not visible. It rises and sets with the Sun. Waxing Phases (increasing illumination): Waxing Crescent: A small sliver of light is visible. First Quarter: Half of the Moon’s face is lit and visible. Waxing Gibbous: More than half is lit. Full Moon: The Earth is between the Moon and the Sun. The entire face of the Moon visible from Earth is illuminated. It rises at sunset and sets at sunrise. Waning Phases (decreasing illumination): Waning Gibbous Third Quarter: Again, half is lit, but the opposite half from the first quarter. Waning Crescent

Question 27

why does this happen?

Answer 27

As the Moon orbits Earth, we see different portions of its sunlit side: Always half lit by the Sun. Our perspective changes throughout its 29.5-day orbit.

Question 28

New moon

Answer 28

When the moon is in the direction of the sun The only time a solar eclipse can happen Unlit side faces the Earth Rises with the sun and sets with the sun Not generally visible (even not visible at night)

Question 29

Waxing crescent

Answer 29

(means getting bigger) 3-4 days after the new moon. Rises around 3 hours after the sun. Sets around 3 hours after the sun. Easiest to see right after sunset

Question 30

Waxing (first) quarter

Answer 30

1 week after the new moon Moon is lit from the side Rises around 6 hours after the sun Sets around 6 hours after the sun Easiest to see at night before midnight

Question 31

Waxing gibbous

Answer 31

10 - 11 days after the new moon. Rises around 9 hours after the sun. Sets around 9 hours after the sun. Easiest to see at night – but may set before sunrise

Question 32

Full moon

Answer 32

Two weeks after the new moon. The only time a Lunar eclipse can happen. Rises around 12 hours after the sun rises. Sets around 12 hours after the sun sets. Visible most of the night

Question 33

Waning gibbous

Answer 33

10-11 days before the next new moon. Rises around 3 hours after the sun sets. Sets around 3 hours after the sun rises. Rises before midnight, and is up the rest of the night and morning

Question 34

Waning (third) quarter

Answer 34

(aka last quarter) Around a week before the next new moon Rises around 6 hours before the sun rises Sets around 6 hours before the sun sets Rises around midnight, and is up the rest of the night and morning

Question 35

Waning crescent

Answer 35

2-4 days before the next new moon Rises around 3 hours before the sun rises. Sets around 3 hours before the sun sets. Easiest to see before sunrise

Question 36

New moon

Answer 36

When the moon is in the direction of the sun. The only time a Solar eclipse can happen. Unlit side faces the Earth. Rises with the sun and sets with the sun. Not generally visible

Question 37

Distinguish between lunar and solar eclipses;

Answer 37

Lunar Eclipse What it is: Occurs when Earth passes directly between the Sun and the Moon, so that Earth's shadow falls on the Moon. What we see: Depending on alignment, we can observe: Total lunar eclipse – the Moon passes entirely into Earth’s umbra (full shadow). Partial lunar eclipse – only part of the Moon enters the umbra. Penumbral lunar eclipse – the Moon passes only through Earth’s penumbra (partial shadow), causing a subtle shading. Visibility: Can be seen by anyone on the nighttime side of Earth, making lunar eclipses more commonly observed. Solar Eclipse What it is: Occurs when the Moon passes directly between Earth and the Sun, so that the Moon's shadow falls on Earth. What we see: Depending on where you are within the Moon's shadow: Total solar eclipse – the Sun is completely blocked by the Moon (seen from within the umbra). Partial solar eclipse – only part of the Sun is obscured (seen from the penumbra). Annular eclipse – occurs when the Moon is farther from Earth and appears smaller, so it leaves a ring of sunlight. Visibility: Only visible from a narrow path on Earth, making them rarer to see from any one location.

Question 38

key causes of eclipses

Answer 38

Eclipses only occur when the Sun, Earth, and Moon are perfectly aligned in a straight line This alignment happens only at certain times when the Moon is at or near one of the two points in its orbit where it crosses the ecliptic plane—called nodes.

Question 39

Explain why there is not a solar eclipse at every new moon and why solar eclipses are seen only from some parts of Earth

Answer 39

Solar eclipses do not happen at every new moon because the Moon’s orbit is tilted about 5° relative to Earth’s orbit around the Sun (called the ecliptic plane) --this means the Moon usually passes above or below the Sun in the sky when it's new, so its shadow misses Earth entirely Eclipses can only occur when the new moon happens close to one of the two points where the Moon’s orbit crosses the ecliptic plane—these are called the nodes --the times of year when the Sun, Earth, and Moon can line up with the nodes are called eclipse seasons, which occur only about twice a year A solar eclipse occurs when the Moon’s shadow falls on Earth—but that shadow is small -the Moon’s full shadow (umbra) only covers a narrow path on Earth, up to about 270 km wide -if you're not in that path, you won’t see totality -meanwhile, a partial shadow (penumbra) covers a wider region where people see a partial eclipse --since the Earth is rotating and the Moon is moving in its orbit, the shadow races across Earth at high speed, so only a limited part of the globe is under the eclipse path at any given time