Guides
Astronomy guides
What lies behind sunrise, lunar phases or right ascension? These guides explain the celestial mechanics that Apphemeride calculates for you, clearly, accurately and with images from the app. For values specific to your location, use Apphemeride.
Sun and daylight
Rise and set, twilight, the Sun’s position and the seasons.
Sunrise and sunset explained
Sunrise and sunset mark the moments when the Sun’s upper edge meets an ideal horizon. Atmospheric refraction lifts its …
Civil, nautical and astronomical twilight explained
Twilight is the transition when the Sun is below your horizon but still illuminates the upper atmosphere. Its stages …
Golden hour and blue hour explained
Golden hour is the period around sunrise or sunset when low sunlight can create warm, soft illumination. Blue hour …
The Sun’s position: azimuth and altitude
Two angles describe the Sun’s position. Azimuth gives its direction around the horizon, measured from north through …
Day length: why daylight changes through the year
Day length is the interval between sunrise and sunset. It depends on latitude and season because Earth’s tilted axis …
Equation of time and apparent solar time explained
The equation of time is the difference between apparent solar time, as shown by a sundial, and uniformly running mean …
The analemma: why the Sun traces a figure of eight
Record the Sun’s position from one location at the same uniform clock time through a year and its positions form an …
Solstices and equinoxes: the seasons explained
Solstices and equinoxes are four precise instants in the Sun’s apparent annual path. At the solstices it reaches its …
Midnight sun and polar night explained
Midnight sun means the Sun remains above the horizon through a full day; polar night means it never rises during that …
Sunspots and the solar cycle explained
Sunspots are cooler regions of the solar surface where strong magnetic fields suppress heat transport. They look dark …
Moon
Phases, rise times, distance, libration and earthshine.
Moonrise and moonset: why the times change
Moonrise is roughly fifty minutes later from one day to the next on average, because the Moon moves eastwards in its …
Moon phases: from new moon to full moon
Lunar phases change because the Sun illuminates approximately half the Moon and we see that hemisphere from different …
Full moon names and their cultural origins
Names such as wolf moon and strawberry moon come from regional North American traditions and later almanac publishing. …
Supermoons and micromoons: size and distance
A supermoon is a popular name for a full moon near perigee, the closest part of its orbit. A micromoon lies near …
Lunar libration: the Moon’s apparent rocking
Libration is the apparent rocking that lets us look a little beyond different lunar edges at different times. Orbital …
Lunar occultations: when the Moon hides a star
A lunar occultation happens when the Moon passes in front of a star or planet. A star can disappear almost instantly …
Earthshine: the glow on the Moon’s dark side
Earthshine is sunlight reflected first by Earth towards the lunar night side and then by the Moon back to us. It …
Planets
When and where to observe the planets.
Planet visibility: what makes a planet observable?
Mercury, Venus, Mars, Jupiter and Saturn are the five planets routinely visible to the unaided eye. Their separation …
Opposition, conjunction and quadrature explained
Opposition, conjunction and quadrature describe a planet’s position relative to the Sun as seen from Earth. An outer …
Greatest elongation of Mercury and Venus explained
Greatest elongation is the instant when Mercury or Venus reaches its largest angular separation from the Sun as seen …
Retrograde motion: why planets seem to turn back
Retrograde motion is an apparent reversal against the background stars: a planet temporarily moves westward instead of …
Saturn’s rings: tilt, opening and visibility
Saturn’s ring opening is the angle at which we see its ring plane. The planet’s tilted axis keeps roughly the same …
Jupiter’s Galilean moons: positions and events
Jupiter’s four major moons can appear as tiny points beside the planet in well-supported binoculars. Their orbits range …
Mercury and Venus transits across the Sun
During a transit, Mercury or Venus crosses the Sun’s disk as a small dark circle. It requires inferior conjunction …
Eclipses
Solar and lunar eclipses, calculated precisely in advance.
Solar eclipses: paths, contacts and obscuration
A solar eclipse occurs when a new moon aligns closely enough with the Sun to cast its shadow on Earth. A narrow region …
Lunar eclipses: phases, colour and visibility
A lunar eclipse occurs when a full moon passes through Earth’s shadow. It is visible from places where the Moon is …
Eclipse contact times: C1–C4 and P1–U4 explained
Solar-eclipse contacts mark the key local moments: C1 begins the overlap, C2 and C3 bound totality or annularity, and …
Annular and hybrid solar eclipses explained
A central eclipse’s type depends on the apparent Moon-to-Sun diameter ratio. A large enough Moon can cover the …
Sky and stars
Orientation, brightness, catalogues and meteor showers.
Understanding the night sky above your location
The stars you can see depend on latitude, date and time. Earth’s rotation makes the sky appear to turn, its orbit …
Finding constellations: orientation in the sky
A constellation is an officially defined region of the sky; eighty-eight cover the whole celestial sphere. Familiar …
Find a star by HIP, HD or TYC catalogue number
A catalogue identifier names an entry unambiguously within its source. HR refers to bright-star catalogues, HIP to …
Stellar magnitude: brightness, colour and distance
Stellar brightness is expressed in magnitudes: smaller numbers mean brighter objects, and a difference of five …
Meteor showers: radiants, ZHR and observed rates
A meteor shower occurs when Earth crosses a stream of debris left by a comet or asteroid. The visible count depends on …
Sky brightness and darkness for stargazing
Astronomical night begins when the Sun is more than eighteen degrees below the horizon. Whether it occurs depends on …
Coordinates and time
The language of positional astronomy.
Celestial coordinates: three systems explained
Celestial coordinates describe direction using two angles and a reference plane. Horizontal coordinates use your …
Right ascension and declination explained
Right ascension and declination are the sky’s counterparts to longitude and latitude. Declination measures north or …
J2000, JNow, precession and nutation explained
J2000 is a standard reference associated with the mean equator and equinox at the beginning of 2000. JNow commonly …
Sidereal time: a clock for the rotating sky
Local sidereal time identifies the right ascension crossing your local meridian, using a compatible reference …
Atmospheric refraction and horizon dip explained
The atmosphere bends light and makes celestial objects appear higher, with a typical horizon effect of around …
Julian Date, UT, TT and Delta T explained
Julian Date counts days continuously rather than through months and leap-year rules. Delta T is the difference between …
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