Reading the night sky
The sky turns 15° every hour, Polaris sits at the same height as your latitude, and the stars rise four minutes earlier each night. Set any place, date and time in the 3D sky dome, with 525 real stars from the Yale Bright Star Catalogue and approximate planet positions.
About this course
The sky turns 15° every hour, Polaris sits at the same height as your latitude, and the stars rise four minutes earlier each night. Those three facts are the course in miniature. You set any place, date and time in a 3D sky dome with 525 real stars from the Yale Bright Star Catalogue, and the dome shows how the pieces fit: altitude is measured up from the horizon, and azimuth runs round the horizon from north through east.
The six units start with the sky as a dome, then Polaris and your latitude. Look north from Korea and there is a star almost exactly at the northern pole. At Seoul the pole is 37.6° up, and stars close enough to it circle without ever touching the horizon. Constellations and star names comes next. In 1603 Johann Bayer began labelling stars in each constellation with Greek letters, and many bright stars carry older names, mostly Arabic in origin, such as Aldebaran, Altair, Deneb and Betelgeuse. In Orion, NASA Space Place points out, the stars in one figure are usually not connected at all. The brightness unit uses the magnitude scale, where smaller numbers are brighter and the idea goes back to Ptolemy's second-century catalogue.
Why the sky changes through the year explains that at 9 p.m. in January Orion stands high in the south and in July Scorpius and the Summer Triangle have taken its place; the stars have not moved, Earth has. The last unit finds the planets, which are always near the ecliptic. The model uses JPL's Keplerian elements for 1800 to 2050, and for 18 October 2026 it puts the Sun, Mercury and Mars within 0.01° of JPL's full Horizons ephemeris.
Before you start
No equipment is needed: the 3D sky dome lets you set any place, date and time. The brightness unit uses the magnitude scale, which Stars introduces, and knowing your own latitude helps for the Polaris unit. For real skies, the course suggests a fist at arm's length as a rough 10° ruler.
When you finish
Afterwards you can find altitude and azimuth for a point in the sky, explain why the sky turns 15° an hour, and find Polaris at a height equal to your latitude, about 37.6° from Seoul. You can tell constellations from asterisms, compare brightness by magnitude, explain why stars rise 4 minutes earlier each night, and spot Venus as an evening or morning star.
- 1The sky as a dome: horizon, zenith and the turning skyThe whole sky turns once in 23 hours 56 minutes, about 15° every hour.
- 2Polaris and your latitudeFrom Seoul, at latitude 37.6° north, Polaris stands about 37.6° above the northern horizon.
- 3Constellations and star namesThe IAU divides the whole sky into 88 constellations, agreed in 1922, with boundaries published in 1930.
- 4Star brightness and colourFive magnitudes is exactly a factor of 100 in brightness, so Sirius looks about 25 times brighter than Polaris.
- 5Why the night sky changes through the yearEach star rises about 4 minutes earlier every night, which adds up to 2 hours a month.
- 6Finding the planetsVenus never strays more than about 47° from the Sun, so it is always an evening or a morning star.
Sources for this course (20)
- Yale Bright Star Catalogue 5th Revised Ed. (Hoffleit and Warren 1991, CDS V/50, cdsarc.cds.unistra.fr/ftp/V/50/, NASA HEASARC BSC5P)
- public domain
- IAU The Constellations (iau.org/public/themes/constellations)
- IAU Catalog of Star Names, WGSN (iau.org/public/themes/naming_stars, list pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt)
- US Naval Observatory sidereal time formula (aa.usno.navy.mil/faq/GAST)
- NASA Glenn "Telling Time by the Stars" (grc.nasa.gov/WWW/k-12/Numbers/Math/Mathematical_Thinking/telling_time_by_the_stars.htm)
- NASA Space Place "What Are Constellations?" (spaceplace.nasa.gov/constellations) and "How Long Is One Day on Other Planets?" (spaceplace.nasa.gov/days)
- NASA Earth Fact Sheet (nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html)
- JPL Approximate Positions of the Planets, Standish and Williams (ssd.jpl.nasa.gov/planets/approx_pos.html)
- spot-checked against JPL Horizons for 2026-10-18
- IAU Office of Astronomy for Education glossary "Zodiac" (astro4edu.org/resources/glossary/term/391)
- Sky and Telescope "A Sign of the Times" (skyandtelescope.org/astronomy-news/observing-news/a-sign-of-the-times)
- EarthSky "Sun enters Ophiuchus" (earthsky.org/astronomy-essentials/sun-in-ophiuchus-november-30-to-december-18)
- NASA StarChild "Why is Polaris the North Star?" (starchild.gsfc.nasa.gov/docs/StarChild/questions/question64.html)
- IAU press release iau1603 on the WGSN and 227 star names (iau.org/news/pressreleases/detail/iau1603)
- Huntington Library on Bayer's Uranometria, 1603 (huntington.org/uranometria)
- Wikipedia Apparent magnitude for Pogson 1856 (cross-check)
- EarthSky "Can you see Canopus" (earthsky.org/brightest-stars/few-know-the-second-brightest-star-canopus)
- Seogwipo Astronomy Science Center notices on Canopus (jejumaeil.net/news/articleView.html?idxno=112240)
- KASI (kasi.re.kr) for Korean star names.
Checked: 2026-10-18