Explore the guides

Search all 45 guides, narrow them by category, or show only the ones you have not logged yet.

Search

CategoryAll categoriesMoon Phases and MotionEclipsesSeasons and the Sun’s PathCalendars and ClocksWatching the Sky

LogbookAll guidesNot logged yetLogged

45 of 45 guides

• New MoonAt new Moon the Moon is in the same part of the sky as the Sun, and the side facing Earth is dark.
• Waxing Crescent MoonA thin silver curve, low after sunset.
• First Quarter MoonHalf of the Moon's disk is bright, yet the name says quarter.
• Waxing Gibbous MoonA waxing gibbous Moon is more than half lit and growing toward full.
• Full MoonThe full Moon is the Moon facing the Sun's opposite side of the sky.
• Waning Gibbous MoonA waning gibbous Moon is more than half lit and shrinking.
• Last Quarter MoonAt last quarter the Moon shows half its disk lit, and the lit part is shrinking.
• Waning Crescent MoonThe waning crescent is the last phase before New Moon, a thin curve of light that keeps shrinking.
• Why We See the Same Face of the MoonThe Moon does rotate, but only once for each trip around Earth, so one side always points our way.
• Total Solar EclipseA total solar eclipse is the Moon's darkest shadow, the umbra, landing on Earth.
• Annular Solar EclipseAn annular solar eclipse happens when the Moon is at or near its farthest point, so it looks smaller than the Sun and leaves a bright ring.
• Partial Solar EclipseA partial solar eclipse happens when only the Moon's outer shadow, the penumbra, reaches Earth, so the Sun is covered in part and never fully.
• Total Lunar EclipseA total lunar eclipse happens when the entire Full Moon passes into the darkest part of Earth's shadow.
• Partial Lunar EclipseA partial lunar eclipse happens when only part of the Full Moon passes through the dark inner part of Earth's shadow.
• Penumbral Lunar EclipseA penumbral lunar eclipse happens when the Full Moon passes through only the faint outer part of Earth's shadow.
• Why Most New Moons Bring No Solar EclipseThe Moon's orbit is tilted about five degrees, so at most New Moons its shadow passes above or below Earth.
• Why Most Full Moons Bring No Lunar EclipseMost Full Moons pass above or below Earth's shadow because the Moon's orbit is tilted about 5 degrees.
• Watching Eclipses SafelyOnly the brief total phase of a total solar eclipse is safe to watch without eye protection.
• Summer SolsticeAt the June solstice, Earth's axis leans the Northern Hemisphere as far toward the Sun as it ever goes, so the noon Sun stands highest there.
• Winter SolsticeAt the December solstice, Earth's axis leans the Northern Hemisphere as far from the Sun as it ever goes, so the day is shortest and the noon Sun...
• Spring EquinoxAt the March equinox the Sun stands over the equator and day and night come out nearly equal, though not exactly.
• Autumn EquinoxThe September equinox is one moment, and it lands on different calendar days in different time zones.
• Earth’s Axial TiltEarth's axis leans about 23.4 degrees from the perpendicular to its orbit.
• Why Day Length ChangesDay length is set by two things, where you are and what day it is.
• The Midnight SunThe midnight Sun happens north of the Arctic Circle, where Earth's tilt keeps the Sun above the horizon for a whole day.
• The Sun’s Height at NoonThe Sun's height at noon follows one rule: 90 degrees minus your latitude at the equinox, then up or down by the tilt at the solstices.
• Perihelion and AphelionEarth is closest to the Sun around January 3, in the middle of northern winter.
• The Solar Calendar and the Tropical YearA solar calendar keeps its dates in step with the seasons.
• Lunar CalendarsA lunar calendar counts months from one cycle of Moon phases to the next.
• Leap YearsYears divisible by 4 are leap years, except century years that are not divisible by 400.
• Time ZonesEarth turns 15 degrees of longitude every hour, and time zones were built on that.
• The International Date LineThe International Date Line is the imaginary line where one calendar day ends and the next begins.
• Solar NoonSolar noon is the moment the Sun crosses the north-south line above your head and reaches its highest point of the day.
• The Sidereal Day and the Solar DayA sidereal day is one turn of Earth measured against the stars, about 23 hours 56 minutes 4 seconds.
• Two Lengths of a Lunar MonthThe sidereal month, about 27.32 days, is the Moon's return to the same star.
• The Equation of TimeThe equation of time is the difference between sundial time and clock time, and it reaches as much as about 16 minutes.
• When the Moon RisesThe Moon rises about 50 minutes later each day, so over one cycle of phases moonrise slides around the whole clock.
• When the Moon SetsA Moon sets roughly half a day after it rises, and each day it sets about 50 minutes later.
• The Moon in DaytimeThe Moon spends nearly as many hours above the horizon in daylight as at night.
• EarthshineOn a crescent Moon the unlit part can glow faintly.
• TwilightTwilight is the glow before sunrise and after sunset.
• Sunrise and SunsetAlmanacs define sunrise and sunset by the upper edge of the Sun, not its center.
• Moonlight and Dark SkiesMoonlight and city lights both wash out faint stars.
• The Moon with BinocularsBinoculars give the Moon craters and ridges.
• Keeping a Sky LogNASA and JPL journals ask for little: the date, the time and a drawing, kept for a month.