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Science Astronomy Light pollution Salesforce Tower

Vol. Science · Astronomy · Luna

Moon
phases

Set a date and a clock, drag the dial, or press Now. The desk shows the Moon’s phase at that moment — name, age, and the geometric fraction of the disk in sunlight. Pair with tides (new and full line up with spring tides; the quarters are neap), the Campbell / Bay Area local sky, and the light pollution desk when planning faint-object nights.

Phase instants follow Jean Meeus, Astronomical Algorithms (2nd ed., ch. 49). The mean synodic month for the year 2000 is 29.530588 days (Fred Espenak, NASA GSFC). The real Moon also librates — a peek past the mean face, on its own desk. Its orbit is tilted about 5°, so a new or full Moon is not an eclipse. That geometry has its own desk: Eclipses. Why the phase cycle is longer than one orbit against the stars is the synodic month desk — a schematic, not this calendar.

Interactive · Phase at a moment

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Drag around the Moon · Sun light from the right

Handy numbers

Synodic month
29.530588 days in the year 2000 — new Moon to new Moon. One month can run about 29.26 to 29.80 days.
Sidereal month
~27.3 days — one orbit relative to the stars; shorter than the phase cycle.
Mean distance
~384,400 km · about 30 Earth diameters · light travel ~1.3 s.
Diameter
~3,475 km · roughly 1/4 Earth’s · gravity ~1/6 g on the surface.
Terminator
Day/night line on the Moon — best contrast for craters near first/last quarter.
Earthshine
Near new, the dark disk can glow faintly from sunlight reflected off Earth.

Observing from the South Bay

Moon vs deep sky

Keep going

The month

Synodic

Why new Moon to new Moon is longer than one orbit against the stars. A schematic, not this dial.

Bulges

Tides

Same phases, different question: two ocean bulges, spring at new and full, neap at the quarters.

Inner orbit

Venus

Crescent through gibbous for a different reason — Venus orbits inside Earth, not around us.

01

Solar map

Sun through Neptune schematic — click planets for AU and plain-English cards.

02

Local sky

Campbell / Bay Area orientation chart for planning sessions.

03

Stars

OBAFGKM colors and life phases — Sun-like vs high-mass paths.

04

Nearest stars

Proxima through the local neighborhood scale.

05

Exoplanets

Discovery methods and notable confirmed worlds.

06

Galaxy

Milky Way arms and Sol on the Orion Spur.

07

Day for Night

Salesforce Tower’s crown — public art and South Bay skyglow glance.

Sources

Cited, not invented

Educational schematic. The clock is your local civil time; the zone name is the one the browser reports. Phase names follow common Northern-hemisphere naming (waxing lit on the right). Distances elsewhere on the page round public NASA / USGS educational figures. Not for navigation or eclipse prediction. Earthrise preview image: NASA / Bill Anders (public domain).