Astro 101: The Sun and Its Neighbors - Moon Phases and Eclipses
Moon Phases and Eclipses
Introduction to the Earth-Moon-Sun System
Recap: Last lecture covered Earth's rotation and orbit explaining seasons, including the tipped axis of the Earth relative to the ecliptic plane.
Tonight's Focus: Understanding how the Moon's orbit around the Earth helps explain moon phases and eclipses.
Complexity: The system is intricate, involving:
Earth's rotation with a tipped axis.
Earth orbiting the Sun.
Moon orbiting the Earth, with its orbit also tipped relative to the Earth's orbital plane.
Moon's rotation synchronized with its orbit around Earth.
Varying sizes and locations of celestial bodies.
Methodology: A simulation is used to visualize these complex interactions, with an acknowledgment that the model will have inaccuracies that will be discussed later.
Initial Model and Observations
Simplified System: The initial model includes the Sun, Earth, and Moon orbiting the Earth.
Moon Illumination: Only half of the Moon is illuminated by the Sun at any given time; the other half is in shadow.
Moon Phases: The visible phase of the Moon from Earth depends on the observer's position relative to the illuminated side of the Moon.
Eclipses Introduced:
Lunar Eclipse: Occurs when the Moon enters the Earth's shadow.
Solar Eclipse: Occurs when the Moon blocks the light from the Sun, casting a shadow on Earth.
Distinction: Moon phases are caused by viewing the illuminated portion of the Moon from different angles, whereas eclipses are caused by shadows (either Earth's shadow on the Moon or Moon's shadow on Earth).
Problem with Initial Model: The initial simulation incorrectly showed a lunar eclipse every full moon and a solar eclipse every new moon.
Month Definition: A
Moon Phases and Eclipses
Introduction to the Earth-Moon-Sun System
Recap: Last lecture covered Earth's rotation and orbit explaining seasons, including the tipped axis of the Earth relative to the ecliptic plane.
Tonight's Focus: Understanding how the Moon's orbit around the Earth helps explain moon phases and eclipses.
Complexity: The system is intricate, involving:
Earth's rotation with a tipped axis, which is the primary cause of day and night and contributes to seasonal variations due to its constant tilt.
Earth orbiting the Sun in an elliptical path, taking approximately days, defining a year.
Moon orbiting the Earth, with its orbit also significantly tipped relative to the Earth's orbital plane (ecliptic plane) by about . This tilt is crucial for understanding why eclipses do not occur monthly.
Moon's rotation synchronized with its orbit around Earth, resulting in the same side of the Moon always facing Earth (tidal locking).
Varying sizes and locations of celestial bodies: The Sun is vastly larger than Earth, which is much larger than the Moon, and their distances vary dynamically.
Methodology: A simulation is used to visualize these complex interactions, with an acknowledgment that the model will have inaccuracies, particularly concerning the scale of distances and sizes, that will be discussed later.
Initial Model and Observations
Simplified System: The initial model includes the Sun, Earth, and Moon orbiting the Earth in a single, simplified plane to illustrate basic principles.
Moon Illumination: Only half of the Moon is illuminated by the Sun at any given time; the other half is in shadow. This illuminated portion is always constant, but our view of it changes.
Moon Phases: The visible phase of the Moon from Earth depends on the observer's position and the angle at which we view the illuminated side of the Moon as it orbits Earth. As the Moon moves in its orbit, the proportion of its sunlit face visible from Earth changes, leading to the different phases (e.g., new moon, crescent, quarter, gibbous, full moon).
Eclipses Introduced:
Lunar Eclipse: Occurs when the Moon enters the Earth's shadow. This can only happen during a full moon.
Solar Eclipse: Occurs when the Moon blocks the light from the Sun, casting a shadow on Earth. This can only happen during a new moon.
Distinction: Moon phases are caused by viewing the illuminated portion of the Moon from different angles, whereas eclipses are caused by shadows (either Earth's shadow falling on the Moon or the Moon's shadow falling on Earth).
Problem with Initial Model: The initial simulation incorrectly showed a lunar eclipse every full moon and a solar eclipse every new moon. This illustrates a common misconception and highlights a key simplification.
To correct this, we must consider the Moon's orbital tilt: The Moon’s orbit around the Earth is not in the same plane as the Earth’s orbit around the Sun (known as the ecliptic plane). Instead, it is tilted by approximately relative to the ecliptic. Eclipses only occur when the Moon is crossing the ecliptic plane (at specific points called nodes) and it is either a new moon (for a solar eclipse) or a full moon (for a lunar eclipse). Because of this tilt, most new moons and full moons occur when the Moon is either above or below the ecliptic plane, meaning its shadow misses Earth (for solar eclipses) or Earth's shadow misses the Moon (for lunar eclipses). This orbital inclination makes eclipses relatively rare events, occurring only a few times a year, compared to the monthly cycle of moon phases.
Month Definition: A synodic month (also known as a lunar month) is the time it takes for the Moon to complete one cycle of phases, from one new moon to the next. This period averages about days. It is longer than the sidereal month (approximately days), which is the time it takes for the Moon to complete one orbit around the Earth with respect to the distant stars, because the Earth also moves in its orbit around the Sun during this time, requiring the Moon to travel a bit further to realign with the Sun and Earth to present the same phase.