Problem 8 - An Introduction to Physical Science

Question

Consider a person in the United States who sees the last-quarter phase of the Moon. (a) Which side of the Moon is illuminated? (b) What phase does an observer in Australia see at the same time, and which side is bright?

Step-by-Step Solution

Answer

(a) In the USA, the left side of the Moon is illuminated. (b) In Australia, the same last-quarter phase is seen with the right side illuminated.

Step 1: Understand the Moon's Last-Quarter Phase

When the Moon is in its last-quarter phase, it means that half of the Moon's disk is illuminated and the other half is in darkness. During this phase, the Moon is positioned such that it is 90 degrees away from the Sun as seen from Earth.

Step 2: Determine the Illuminated Side in the USA

In the Northern Hemisphere (e.g., the United States), during the last-quarter phase, the left half of the Moon is illuminated, while the right half is dark. This occurs because the Moon is moving towards a new phase, and observers see the western side lit.

Step 3: Determine the Phase Seen in Australia

Observers in Australia, located in the Southern Hemisphere, will see the same phase of the Moon but from a different orientation. During the last-quarter moon phase, they too will see half of the Moon illuminated. However, because they are in the Southern Hemisphere, the orientation is flipped vertically compared to the Northern Hemisphere.

Step 4: Identify Which Side is Bright in Australia

In Australia, the illuminated (bright) side of the last-quarter Moon appears to be the right half. This is due to the Southern Hemisphere orientation, making the western side of the Moon appear on the right.

Key Concepts

last-quarter phase

The last-quarter phase of the Moon is a fascinating celestial event. During this phase, the Moon is approximately at a right angle to the Earth and the Sun. This geometric position means that one half of the Moon's visible surface is lit up by the Sun, while the other half is in shadow. The term "last-quarter" can be a bit confusing since we see half of the Moon's surface illuminated.

It occurs after the full moon when the Moon is waning, or reducing in visible size. The illuminated part represents the Moon graphically moving through its orbit towards a new moon. Essentially, at this stage, the Moon is three-quarters of the way through its orbit around Earth.

It's a great time for sky watchers and enthusiasts to observe the Moon, as the irradiated side is often more pronounced and distinct. With a backyard telescope or even binoculars, you can see detailed contrasts on the surface, perfect for understanding lunar geography.

Northern Hemisphere

In the Northern Hemisphere, such as the United States, observing the Moon in the last-quarter phase offers a unique perspective. When a person looks up at the sky, they will notice that the left side of the Moon is illuminated. The right side remains in darkness.

This configuration is because, from this vantage point, observers see the Moon moving progressively toward the new moon phase. In simpler terms, the western half of the Moon is illuminated, a noticeable shift as the sunlit portion decreases.

• The left side is bright due to the positioning of the Earth, Moon, and Sun.
• The Moon rises around midnight and sets around noon, providing night owls perfect viewing.
• It's the inverse of what occurs during the first-quarter phase when the right side would be lit up.

Observing this phase can truly enhance one’s understanding of the Moon's cycle and the concept of earthshine and lunar phases.

Southern Hemisphere

For observers located in the Southern Hemisphere, like in Australia, the perception of the Moon's last-quarter phase takes a different turn. Because they look up from the opposite side of the equator, the orientation of the Moon’s brightness is flipped. So, while the same amount of the Moon is illuminated, the right side appears bright instead of the left.

Therefore, when an Australian looks at the last-quarter moon, the western half that is illuminated appears on the right side in their sky:

• This is all due to the different orientation offered by their hemispherical location on Earth.
• The moon's path appears to travel right to left, unlike the left to right path seen in the Northern Hemisphere.
• This knowledge helps understand how the Moon’s phases are a universal phenomenon but appear differently from various global vantage points.

This highlights the wonder of astronomy as it connects people around the world in the shared experience of celestial events, while still allowing for a view unique to their place on Earth.