HO

Charting the Heavens II — Moon Phases, Eclipses, and Distance

Moon Basics

  • The Moon orbits the Earth; it does not shine on its own but reflects sunlight.
  • The Moon's orbit is tilted with respect to the Earth's orbit around the Sun.
  • First, you need to know these things about the Moon: understanding its orbit, illumination, and scale is foundational for all following topics in charting the heavens.

Scale of the Earth and Moon

  • Typical Earth–Moon distance (average):
    • d \,\approx\, 384{,}400\ \text{km}
    • \approx\, 238{,}555\ \text{miles}
  • There is a lot of space in space; the Moon is far away relative to human-scale distances, which leads to noticeable angular sizes despite its small physical size.
  • Distance figure is often paired with reminders like "with about 8{,}030\ \text{km} or 4{,}990\ \text{miles} to spare" to give a sense of scale in context.

Phases of the Moon

  • Why Moon phases occur: the Sun shines on the Moon; half of the Moon is always lit by the Sun; the portion we see lit changes as the Moon orbits Earth.
  • Phases are the changing views of the sunlit portion, not to scale in diagrams.
  • The major Moon phases (in order) include: New, Waxing Crescent, First Quarter, Waxing Gibbous, Full, Waning Gibbous, Third Quarter, Waning Crescent, then back to New.
  • For reference, a typical phase sequence diagram may show labels such as Waxing Crescent, First Quarter, Waxing Gibbous, Full, Waning Gibbous, Third Quarter, Waning Crescent, New.

Eclipses: General Concepts

  • Eclipses occur when Earth, Moon, and Sun form a line (alignment) in a way that the shadow or light obstruction can affect what we see from Earth.
  • Eclipses don’t happen every month because the Moon’s orbit is inclined relative to the Earth–Sun plane (not in the same flat plane as the Sun–Earth line).
  • Two broad eclipse types:
    • Lunar eclipse: Earth blocks sunlight from reaching the Moon (Earth is between Sun and Moon).
    • Solar eclipse: Moon blocks sunlight from reaching the Earth (Moon is between Earth and Sun).

Lunar Eclipses

  • Occur only at Full Moon (Earth is between Moon and Sun).
  • Three types of lunar eclipses:
    • Penumbral lunar eclipse: the Moon passes through Earth’s penumbra (partial shading, subtle shading change).
    • Partial lunar eclipse: part of the Moon passes through the umbra (partial shading of the Moon).
    • Total lunar eclipse: the entire Moon passes through the Earth’s umbra (full shading; the Moon can appear red/orange).
  • Visibility rule: anyone on Earth's nighttime side who can see the Moon during the eclipse can observe the event.
  • Why the Moon appears red during a total lunar eclipse:
    • Earth’s atmosphere filters some sunlight and allows it to reach the Moon’s surface.
    • Blue light is scattered out (causing the blue sky during the day), leaving red/orange light.
    • Some of this remaining light is refracted as it passes through Earth’s atmosphere, contributing to the reddish hue on the Moon.
    • The exact color depends on atmospheric conditions such as dust and clouds; this is similar to the color seen at sunset.
  • Upcoming lunar eclipses visible from the United States:
    • March 3, 2026 (total)
    • August 28, 2026 (partial)

Solar Eclipses

  • Occur when the Moon’s shadow covers part of the Earth; only happens at New Moon (Moon between Earth and Sun).
  • Three main types:
    • Total solar eclipse: the Moon’s umbra fully covers the Sun; corona visible.
    • Partial solar eclipse: only a portion of the Sun is obscured (seen in the penumbra).
    • Annular solar eclipse: the Moon is too far away to fully cover the Sun, so a bright ring (donut) of Sun remains visible around the Moon.
  • Shadow terminology:
    • Umbra: the region of complete shadow where the Sun is fully obscured by the Moon (total eclipse).
    • Penumbra: the partial shadow where only part of the Sun is obscured (partial eclipse).
  • Total solar eclipse specifics:
    • Observers in the umbra see a total eclipse and may observe the solar corona.
    • Observers in the penumbra see a partial eclipse and should not look directly at the Sun.
    • The total eclipse lasts only a few minutes.
    • The path of totality is very narrow and very long: roughly \text{path length} \approx 10{,}000\ \text{miles} and \text{width} \approx 100\ \text{miles}.
  • Great American Eclipses (recent notable totals):
    • 2017 August 21 (total solar eclipse in the U.S.)
    • 2024 April 8 (total solar eclipse in the U.S.)
  • Solar eclipses observed from spacecraft:
    • NASA's EPIC (Earth Polychromatic Imaging Camera) aboard the Deep Space Climate Observatory (DSCOVR) captured Earth-side views of the 2017 eclipse from space.
    • NASA's Solar Dynamics Observatory (SDO) and NASA’s Lunar Reconnaissance Orbiter (LRO) provided additional spacecraft perspectives of the 2017 event.
  • Eclipse effects on weather and environment (2017 example, University of Alabama in Huntsville data):
    • Loss of solar radiation during the eclipse caused temperature drops (e.g., a temporary ~10°F decrease in surface temperatures).
    • Dew point and humidity displayed characteristic changes; data illustrated nighttime-like conditions during peak eclipse hours.
    • Visuals showed temperature, dew point, and relative humidity changes across the eclipse timeline.
  • Annular eclipses:
    • Occur when the Moon is at apogee (farther from Earth) and cannot fully cover the Sun, producing a ring around the Moon.
  • Upcoming solar eclipses (path maps and timelines):
    • Next partial solar eclipse in Texas: 2029-01-14
    • 2021–2040: Series of total, partial, and annular eclipse paths with various global sightings (illustrated by latitude/longitude path maps).
    • 2045: The next major total solar eclipse crossing the continental United States is on 2045-08-12.
    • Additional 2041–2060 path maps show multiple total, annular, and hybrid eclipses across different longitudes.

Distance and Size Measurements

  • Measuring distance to objects in space uses geometric methods:
    • Triangulation: By measuring the baseline distance and the angles to an object, distance can be calculated. Typical description: with a known baseline B and observed angles, distance D can be solved from trigonometric relations.
    • Parallax: Similar concept to triangulation, but based on observing apparent motion of an object against distant background from two vantage points.
  • Converting angular diameter to physical size:
    • If an object has angular diameter \alpha (in radians) and is at distance D, its physical diameter S is approximately
    • S \approx D\,\alpha
    • For more exact relation, using small-angle approximation: \alpha \approx \frac{S}{D} \quad\text{or}\quad S \approx D\,\alpha with \alpha in radians.
    • A more precise relation for larger angular sizes uses S = 2D\tan\left(\frac{\alpha}{2}\right).

Practice and Interactive Review: Name That Feature & Quick Checks

  • Interactive prompts labeled as "Let’s Play Name That Feature!" appear in the material, encouraging identification of lunar/astronomical features (details not provided in the transcript).
  • Quick practice questions (multiple choice) included:
    • If we have a New Moon today, when will we have the next Full Moon?
    • Options: • In about 2 weeks • In about 1 week • In about a month • In about 6 months
    • Correct: In about 2 weeks.
    • Lunar eclipses can occur only during a .
    • Options: • new moon • full moon • first quarter moon • third quarter moon
    • Correct: full moon.
    • We cannot see a New Moon in our sky because _.
    • Options: • it is obscured by the Earth's shadow • a new Moon is quite near the Sun in the sky • no visible sunlight is illuminating the Moon • it is above the horizon during the daytime
    • Correct: a new Moon is quite near the Sun in the sky.

Real-World Relevance and Safety

  • Eclipses offer public outreach and education opportunities, including live viewing and data collection.
  • Safety note for solar eclipses: never look directly at the Sun during the partial phase or when you are not in the path of totality; during a totality, viewing the Sun is momentarily safe, but only while in the umbra and with proper eye protection before and after the total phase.
  • Spacecraft observations provide complementary perspectives, enabling scientists to study Earth’s atmosphere, solar corona, and the Moon’s appearance during eclipses.

Connections to Foundational Concepts

  • Orbital mechanics: understanding the Moon’s orbit inclination explains why eclipses do not happen every month.
  • Light and shadows: phases are a consequence of illumination geometry, while eclipses are shadow geometry involving umbra and penumbra.
  • Measurement science: distance and size estimation rely on triangulation, parallax, and angular-size relationships, foundational to astrometry.
  • Observational astronomy: space-based instruments (EPIC, SDO, LRO) illustrate how multi-perspective data enhances understanding of events like eclipses.