The Shifting Moon: Why Moonrise and Moonset Change

ChatGPT:

The time of the moonrise changes each day because of the moon’s orbit around Earth. The moon moves approximately 13 degrees eastward along its orbit every day, which means it takes longer for the Earth to rotate and bring the moon into view from a particular location.

Here’s why:

1. Earth’s Rotation and Moon’s Orbit: While Earth spins on its axis, completing one rotation every 24 hours, the moon is also orbiting Earth. Each day, the moon moves a little further along in its orbit, which shifts its position in the sky relative to a fixed point on Earth.
2. Daily Delay: Because the moon is orbiting eastward, it rises about 50 minutes later each day. This is due to the extra distance Earth has to rotate to bring the moon back into view as it shifts eastward.
3. Elliptical Orbit: The moon’s orbit around Earth is elliptical, not perfectly circular, meaning that the speed of the moon’s movement changes slightly depending on where it is in its orbit, leading to some variability in the exact time difference from day to day.

This daily shift in moonrise times is why the moon appears at different times throughout the lunar month.

The moon rises in the east and sets in the west for the same reason the Sun and stars do: due to the rotation of the Earth. Earth spins from west to east on its axis, which makes celestial objects (like the moon) appear to rise in the east and set in the west.

Why the Moon Changes Position Every Day:

1. Earth’s Rotation:

• As Earth rotates on its axis once every 24 hours, it causes the moon (and other celestial bodies) to appear to move across the sky from east to west. This daily apparent movement is due to Earth’s rotation.

1. Moon’s Orbit Around Earth:

• The moon is also orbiting Earth in an eastward direction. As it moves along its orbit, it shifts its position relative to Earth.
• The moon moves about 13 degrees eastward each day in its orbit. Because of this, the moon rises roughly 50 minutes later each day.

1. Daily Shift in Moonrise:

• As the moon orbits Earth, it changes its position in the sky each day. This eastward movement means that it takes longer for the Earth to rotate enough for the moon to appear at the horizon, causing the moonrise time to shift later by approximately 50 minutes each day.
• Over a month (about 29.5 days), the moon completes one full orbit around Earth, which explains the repeating cycle of moon phases and the changing positions of moonrise and moonset.

Summary:

• The moon rises in the east and sets in the west due to Earth’s rotation.
• The moon changes position and rises later each day because it is orbiting Earth in the same direction Earth rotates, causing a daily shift of about 50 minutes.

A supermoon occurs when the moon is both full (or new) and at its closest point to Earth in its elliptical orbit, called perigee. This makes the moon appear larger and brighter than usual when viewed from Earth.

Key Points:

1. Elliptical Orbit:

• The moon’s orbit around Earth is not a perfect circle; it’s elliptical. This means the moon’s distance from Earth varies, with a closest point (perigee) and a farthest point (apogee).
• At perigee, the moon is about 14% closer to Earth than at apogee.

1. Full Moon at Perigee:

• A supermoon happens when the full moon coincides with perigee, making it appear larger (up to 14% bigger) and about 30% brighter than a regular full moon.
• The effect is most noticeable when the moon is near the horizon, creating an optical illusion that exaggerates its size.

1. Frequency:

• Supermoons occur about 3-4 times a year. Not every full moon aligns perfectly with perigee, so only those that do are called supermoons.

1. Terminology:

• The term “supermoon” was coined by astrologer Richard Nolle in 1979, but it has since been widely adopted in astronomy.
• The technical term in astronomy for this phenomenon is perigee-syzygy of the Earth-Moon-Sun system.

1. Impact:

• While visually striking, supermoons have minimal physical effects on Earth. They can cause slightly higher tides (known as perigean spring tides) due to the increased gravitational pull, but the difference is small.

In short, a supermoon is a full moon that appears larger and brighter because it occurs when the moon is at its closest point to Earth.

The variation in the time difference for the moonrise, such as the change from 66 minutes to 22 minutes near Toronto, is primarily due to the combination of factors related to the moon’s orbit and the Earth’s rotation. Here’s why this happens:

1. Elliptical Orbit of the Moon:

• The moon follows an elliptical (oval-shaped) orbit around Earth. This means that its speed is not constant; it moves faster when it is closer to Earth (at perigee) and slower when it is farther away (at apogee).
• When the moon is moving faster in its orbit (near perigee), the difference in the moonrise time from one day to the next is smaller, sometimes as little as 22 minutes.
• When the moon is moving slower (near apogee), the time difference is larger, potentially as much as 66 minutes.

2. Lunar Declination:

• The moon’s orbit is tilted relative to Earth’s equator. As the moon moves along its orbit, it changes its declination (its position north or south of the celestial equator).
• When the moon is near the celestial equator, the time difference between consecutive moonrises is smaller. However, when it moves far north or south of the equator, the time difference becomes larger because it appears to rise or set at a steeper angle relative to the horizon.

3. Observer’s Latitude:

• Toronto’s latitude also affects the moonrise time variation. The closer you are to the poles, the more pronounced these variations can be due to the geometry of the Earth’s surface and the angle at which the moon rises.

4. Phases of the Moon:

• The phase of the moon can also impact how much the moonrise time shifts. Around the new moon or full moon, the moon is near the Sun’s position in the sky, and the time difference tends to be more uniform. However, during the first and last quarters, the time shift can be more irregular due to the moon’s position relative to the Earth and Sun.

These factors combine to create variability in how much the moonrise time changes from one day to the next.