Real Astronomy Sky Map
Set your location and the date and time, then watch the night sky as it really appears overhead. The chart projects bright stars from the Yale Bright Star Catalog onto a flat dome using stereographic projection, with constellation lines, Messier deep-sky objects, and the major planets layered on top.
Guided Experiment: Find the Big Dipper from New York at 10pm
Where will the Big Dipper (Ursa Major) appear in the sky from New York at 10pm tonight? Predict whether it will be high or low, and which compass direction to look.
Write your hypothesis in the Lab Report panel, then click Next.
Sky Right Now
- Marsmag 1.1 | alt 26.0°
- Saturnmag 1.0 | alt 39.3°
- Uranusmag 5.8 | alt 18.8°
- Neptunemag 7.9 | alt 40.9°
Controls
Naked-eye sky reaches mag 6 in dark conditions. City skies are 3 to 4.
Data Table
(0 rows)| # | Star | Magnitude | Altitude(deg) | Azimuth(deg) | Total Visible | LST(h) |
|---|
Reference Guide
Equatorial vs Horizontal Coordinates
Stars are catalogued in equatorial coordinates that stay fixed to the sky. Right Ascension (RA) measures position along the celestial equator in hours, and Declination (Dec) measures how far north or south of the celestial equator in degrees.
Observers see the sky in horizontal coordinates that depend on location and time. Altitude is the angle above the horizon and azimuth is the compass direction. The sky map converts equatorial to horizontal using the observer latitude and the Local Sidereal Time.
Here h is altitude, the delta and phi are declination and observer latitude, and H is the hour angle (LST minus RA).
Apparent Magnitude Scale
Magnitude is a logarithmic measure of brightness. Lower numbers mean brighter stars. The scale was inverted historically because Hipparchus ranked the visible stars from 1 (brightest) to 6 (faintest naked-eye).
A difference of five magnitudes corresponds to a brightness ratio of exactly 100. Sirius at magnitude -1.46 is about ten times brighter than Vega at 0.03. A dark rural sky reveals stars down to magnitude 6, a typical city sky only magnitude 3 or 4.
Why Stars Rise and Set
Earth rotates roughly once every 23 hours 56 minutes relative to the stars. That is the sidereal day. The slightly longer solar day of 24 hours accounts for Earth moving along its orbit so the Sun returns to the same place in the sky.
The Local Sidereal Time tells you which line of right ascension is overhead right now. Stars with right ascension near the LST are transiting your meridian. Stars rise about four minutes earlier each day, which is why constellations slowly shift from season to season.
Lambda is the observer longitude in degrees east.
Constellations vs Asterisms
The International Astronomical Union divides the sky into 88 official constellations, each with strict boundaries. Orion, Leo, and Scorpius are full constellations. Every star belongs to exactly one constellation.
An asterism is a pattern that is not a full constellation. The Big Dipper is an asterism inside Ursa Major. The Summer Triangle (Vega, Deneb, Altair) is an asterism that spans three constellations.
The chart draws the iconic line patterns that astronomers use to teach the sky. Toggle them off to see how spotting constellations becomes much harder without the hints.
Light-Year and Parsec
A light-year is the distance light travels in one year, about 9.46 trillion kilometers. A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond and equals about 3.26 light-years.
Sirius is 8.6 light-years away. Betelgeuse is about 550 light-years away yet appears almost as bright because it is a red supergiant. Deneb at 2,600 light-years is one of the most luminous stars visible to the naked eye.
Data Sources
Star coordinates and magnitudes come from the Yale Bright Star Catalog (Hoffleit 1991), a public-domain reference covering every star brighter than magnitude 6.5. We bundle the brightest stars (about magnitude 4 and below) plus enough secondaries to draw the major constellation patterns.
Constellation line geometry follows the International Astronomical Union pattern. Planet positions are a fixed snapshot for 2024-06-21 00:00 UTC, taken from JPL Horizons. Planet motion is not modelled live, so the planet overlay is accurate only near that date.
Full catalog reference: Yale Bright Star Catalog 5th edition (Harvard TDC)
