The sky, explained simply

Terms you will see in Sky Viewer and our almanac APIs: coordinate systems, angular units, time scales, and the 88 IAU constellations. No prior astronomy needed.

• 1. Sky coordinates
• 2. Angles & units
• 3. Time
• 4. Bodies & phenomena
• 5. Zodiac & signs
• 6. 88 constellations
• 7. Quick dictionary

1. Sky coordinates

Think of the sky as a giant sphere around Earth. To say where a star is, we use coordinate systems — like GPS for the celestial sphere.

1.1 Horizontal — Altitude & Azimuth

• Altitude (Alt) — angle above the horizon. 0° = on the horizon, 90° = overhead (zenith).
• Azimuth (Az) — compass direction around the horizon. Often 0° = North, 90° = East, 180° = South, 270° = West.

Alt/Az change every minute as Earth rotates. Sky Viewer draws your local sky this way.

1.2 Equatorial — RA / Dec

• Right Ascension (RA) — celestial “longitude”. Hours (0h–24h) or degrees (0°–360°). 1h = 15°.
• Declination (Dec) — celestial “latitude”: 0° = celestial equator, +90° north pole, −90° south pole.

In Sky Viewer / APIs, RA and Dec are usually in degrees. Sirius ≈ RA 101.3°, Dec −16.7°.

1.3 Ecliptic

• Ecliptic — the Sun’s yearly path (Earth’s orbital plane).
• Ecliptic longitude (λ) — 0°–360°, used for seasons / zodiac signs.
• Ecliptic latitude (β) — distance from the ecliptic plane.

The celestial equator is tilted ~23.4° to the ecliptic — that tilt makes the seasons.

2. Angles & units

Unit	Symbol	Equals	Feel
Degree	°	360° in a circle	Fist at arm’s length ≈ 10°
Arcminute	′	1° = 60′	Moon’s diameter ≈ 30′
Arcsecond	″	1′ = 60″	DE440-class precision
RA hour	h	1h = 15°	Catalogs write 6h 45m…
mas	mas	0.001″	Stellar proper motion

3. Time scales

• UTC — civil world clock.
• TT / TDB — uniform time for ephemerides; differs from UTC by tens of seconds (ΔT).
• ΔT — TT − UT1; Earth’s rotation is uneven.
• JD — Julian Day count; J2000.0 = 2451545.0.
• Sidereal time / LST — clock tied to the stars; LST = RA → object on the meridian.

4. Bodies & phenomena

• Magnitude — smaller number = brighter. Sirius ≈ −1.5; naked-eye limit ≈ 6.
• Rise / set / transit — horizon crossing or highest point.
• Lunar phase / eclipse — new/full Moon; solar & lunar eclipses.
• Refraction — atmosphere lifts objects near the horizon.
• Proper motion / parallax — star drift; distance in parsecs.

5. Zodiac & twelve signs

Twelve signs split λ into 12×30° bands — not the same as uneven IAU constellation borders.

Sign	Latin	λ
Aries	Aries	0°–30°
Taurus	Taurus	30°–60°
Gemini	Gemini	60°–90°
Cancer	Cancer	90°–120°
Leo	Leo	120°–150°
Virgo	Virgo	150°–180°
Libra	Libra	180°–210°
Scorpio	Scorpius	210°–240°
Sagittarius	Sagittarius	240°–270°
Capricorn	Capricornus	270°–300°
Aquarius	Aquarius	300°–330°
Pisces	Pisces	330°–360°

Tropical fixes 0° at the vernal equinox. Sidereal follows the star background (~24° ayanamsa).

6. The 88 IAU constellations

The IAU standardized 88 constellations with fixed borders. Every (RA, Dec) belongs to exactly one.

Abbr	Latin	English name	Notes
And	Andromeda	Andromeda	Andromeda Galaxy (M31)
Ant	Antlia	Air Pump
Aps	Apus	Bird of Paradise	Southern sky
Aql	Aquila	Eagle	Star Altair
Aqr	Aquarius	Water Bearer	Zodiac
Ara	Ara	Altar	Southern sky
Ari	Aries	Ram	Zodiac
Aur	Auriga	Charioteer	Star Capella
Boo	Boötes	Herdsman	Star Arcturus
Cae	Caelum	Chisel
Cam	Camelopardalis	Giraffe	Near NCP
Cnc	Cancer	Crab	Zodiac
CVn	Canes Venatici	Hunting Dogs
CMa	Canis Major	Great Dog	Sirius
CMi	Canis Minor	Little Dog	Procyon
Cap	Capricornus	Sea Goat	Zodiac
Car	Carina	Keel	Canopus
Cas	Cassiopeia	Cassiopeia	W shape
Cen	Centaurus	Centaur	Alpha Centauri
Cep	Cepheus	Cepheus
Cet	Cetus	Whale
Cha	Chamaeleon	Chameleon	Southern sky
Cir	Circinus	Compass
Col	Columba	Dove
Com	Coma Berenices	Berenice's Hair	Star cluster
CrA	Corona Australis	Southern Crown
CrB	Corona Borealis	Northern Crown	Crown arc
Crv	Corvus	Crow
Crt	Crater	Cup
Cru	Crux	Southern Cross	Southern Cross
Cyg	Cygnus	Swan	Deneb
Del	Delphinus	Dolphin
Dor	Dorado	Goldfish	Magellanic Cloud
Dra	Draco	Dragon	Around NCP
Equ	Equuleus	Little Horse
Eri	Eridanus	River
For	Fornax	Furnace
Gem	Gemini	Twins	Castor & Pollux
Gru	Grus	Crane
Her	Hercules	Hercules	Globular M13
Hor	Horologium	Clock
Hya	Hydra	Water Snake	Largest constellation
Hyi	Hydrus	Male Water Snake
Ind	Indus	Indian
Lac	Lacerta	Lizard
Leo	Leo	Lion	Regulus
LMi	Leo Minor	Little Lion
Lep	Lepus	Hare
Lib	Libra	Scales	Zodiac
Lup	Lupus	Wolf
Lyn	Lynx	Lynx
Lyr	Lyra	Lyre	Vega
Men	Mensa	Table Mountain
Mic	Microscopium	Microscope
Mon	Monoceros	Unicorn
Mus	Musca	Fly
Nor	Norma	Carpenter's Square
Oct	Octans	Octant	South celestial pole
Oph	Ophiuchus	Serpent Bearer	Near zodiac
Ori	Orion	Hunter	Belt of three stars
Pav	Pavo	Peacock
Peg	Pegasus	Winged Horse	Great Square
Per	Perseus	Perseus	Perseid meteors
Phe	Phoenix	Phoenix
Pic	Pictor	Easel
Psc	Pisces	Fishes	Zodiac
PsA	Piscis Austrinus	Southern Fish	Fomalhaut
Pup	Puppis	Stern
Pyx	Pyxis	Compass
Ret	Reticulum	Reticle
Sge	Sagitta	Arrow
Sgr	Sagittarius	Archer	Toward Galactic center
Sco	Scorpius	Scorpion	Antares
Scl	Sculptor	Sculptor
Sct	Scutum	Shield
Ser	Serpens	Serpent	Caput & Cauda
Sex	Sextans	Sextant
Tau	Taurus	Bull	Pleiades
Tel	Telescopium	Telescope
Tri	Triangulum	Triangle	Galaxy M33
TrA	Triangulum Australe	Southern Triangle
Tuc	Tucana	Toucan	SMC
UMa	Ursa Major	Great Bear	Big Dipper
UMi	Ursa Minor	Little Bear	Polaris
Vel	Vela	Sails
Vir	Virgo	Maiden	Spica
Vol	Volans	Flying Fish
Vul	Vulpecula	Fox

7. Quick dictionary

• AU — ~150 million km (Earth–Sun).
• DE440 — JPL numerical planetary ephemeris.
• Ephemeris — positions vs time.
• Equinox / Solstice — season markers.
• FOV — camera field of view.
• ICRS / J2000 — modern reference frame / epoch.
• Nutation / Precession — Earth’s axis wobble / slow drift.
• Zenith / Nadir — straight up / straight down.