Astronomical Phenomena

Meteor showers occur when Earth passes through a trail of debris left by a comet (or, rarely, an asteroid) orbiting the Sun. As these tiny particles — typically sand-grain sized — enter our atmosphere at speeds of 11–72 km/s, they heat up through friction and produce streaks of light we call meteors.

The Zenithal Hourly Rate (ZHR) is the theoretical number of meteors a single observer would see per hour under perfect conditions: a dark sky (limiting magnitude 6.5), with the shower's radiant (the apparent point of origin in the sky) directly overhead. Real observed rates are typically 50–70% of the ZHR.

The radiant point is not the physical origin of meteors — it is a perspective effect, like parallel railway tracks converging at the horizon. Meteors enter the atmosphere in roughly parallel paths, and their projected directions appear to diverge from a single point.

Major showers include the Perseids (Aug, parent: 109P/Swift-Tuttle), Geminids (Dec, parent: 3200 Phaethon), and Quadrantids (Jan, parent: 2003 EH1).

The Moon's phases are caused by the changing geometry between the Sun, Earth, and Moon. As the Moon orbits Earth, the fraction of its sunlit hemisphere visible from Earth changes in a predictable cycle.

The synodic month — the period from one New Moon to the next — averages 29.53059 days (29 days, 12 hours, 44 minutes). This is longer than the sidereal month (27.32 days) because Earth moves along its orbit, so the Moon must travel a bit farther to return to the same Sun–Earth–Moon geometry.

Earthshine (sometimes called "the old moon in the new moon's arms") is the faint glow on the unlit portion of a crescent Moon, caused by sunlight reflecting off Earth back to the Moon and then returning to the observer. It is most visible in the days after New Moon.

The Moon's illumination is described by its phase angle (Sun–Moon–Earth angle) and age (days since the last New Moon, 0–29.53).

Eclipses occur when the Sun, Earth, and Moon align closely in three dimensions. A solar eclipse happens at New Moon when the Moon passes between the Sun and Earth; a lunar eclipse happens at Full Moon when the Moon enters Earth's shadow.

Eclipses don't occur every month because the Moon's orbit is tilted ~5.1° to the ecliptic. Eclipses only occur near the nodes — the two points where the Moon's orbit crosses the ecliptic plane.

The umbra is the region of total shadow; the penumbra is the region of partial shadow. A total solar eclipse is visible only within the narrow umbral path (typically ~100–250 km wide), while a partial eclipse is visible over a much wider penumbral region.

Eclipses recur in patterns called the Saros cycle — a period of 18 years, 11 days, and 8 hours (6585.3 days). After one Saros, the Sun, Moon, and nodes return to nearly the same relative positions, producing a similar eclipse shifted ~120° westward (due to the 8-hour fraction).

The gamma value of an eclipse indicates how centrally the Moon's shadow strikes Earth (0 = perfectly central, ±1 = edge of Earth). Values beyond ±1 result in partial or missed eclipses.