How to read a star map: direction, one night, and the changing seasons
Turn around, return a few hours later, or come back months later: the same sky looks different for three different reasons. Start with direction and the horizon, then keep your view fixed while changing the hour and the date.
How does a star map match the sky in front of you?
Choose a city and locate north, east, south, west, the horizon, and the zenith. Azimuth runs clockwise from north; altitude rises from the horizon. Match your facing direction before looking for a star.
Why do stars move during one night?
Earth rotates eastward, so distant stars appear to move westward. Keep your city and facing direction fixed and advance the clock to see their positions change relative to the horizon.
Why does the sky differ at the same hour in another month?
As Earth orbits the Sun, the nighttime side faces a different part of space. Hold the city and local clock time fixed, then change the date to compare the star fields.
The most obvious seasonal change in the month experiment is the brightest stretch of the Milky Way. When and where to look for the Milky Way's centre has its own lesson, recalculated on the same chart.
Find your direction, then watch the sky change
Before finding a star, line up the chart with the direction you face. Dragging changes your view; keeping it still while changing the hour or month reveals two different motions.
Start with the horizon
Face south outdoors. The horizon along the bottom of the chart marks where sky meets ground. East, west, and north buttons change your view; dragging the chart is another way to turn.
Azimuth tells you which way to face: north is 0°, east 90°, south 180°, west 270°. Altitude tells you how far to look up: 0° at the horizon, 90° overhead. Select a bright star and read both numbers.
Hold your direction through one night
Keep the chart facing south and move from 21:00 to 03:00 the next day. Earth rotates, so stars appear to move westward relative to the horizon; one star's azimuth and altitude change.
Some stars rise and set; at some latitudes others circle a celestial pole without setting. A negative altitude puts a target below the geometric horizon at that moment.
Keep the hour and change the month
Hold local 21:00 fixed and change only the date. Earth has moved along its orbit, so the nighttime side faces another star field. This is not a fast-forward of one night.
Sydney and Taipei do not see identical skies in the same month, and their seasons are opposite. The controls name months directly rather than assuming everyone shares a northern summer.
The chart still has real-world limits
This chart uses city-center coordinates and a geometric horizon. Hills, buildings, elevation, and atmospheric refraction can move the apparent boundary at your site.
Daylight and twilight can hide starlight. Clouds, moonlight, and light pollution are not calculated here either. The map helps you find a direction; it cannot promise every star will be visible tonight.
Aim the chart at the sky you are watching
This chart uses a local star catalog and the selected city's coordinates. Dragging changes where you look; hour and month have separate controls. Try the direction buttons first.
City and local 21:00 stay fixed; change only your viewing direction.
City and viewing direction stay fixed; advance this night's clock.
City, direction, and local 21:00 stay fixed; change only the month.
The chart shows geometric positions, not hills, buildings, refraction, or visible brightness. The Sun, the Moon and the seven planets other than Earth are drawn near the ecliptic; Uranus and Neptune sit at or past naked-eye reach, so their labels mark where they are, not that you can see them.
Three experiments to repeat
In Find direction, press east, south, and west, then drag the chart. Compare the view-center azimuth with the horizon labels without changing the date.
In Advance one night, move from 21:00 to 03:00 the next day. Keep facing the same way and watch one bright star's azimuth and altitude change.
In Compare months, keep local 21:00 and direction fixed while comparing January with July. Repeat in Sydney; do not carry northern-hemisphere season names across.
First align the chart with where you stand
The chart projects catalog star positions onto the sky at a city's coordinates and a chosen instant. In the south-facing view the horizon lies below; cardinal labels are fixed reference directions. Dragging changes only where you look.
Azimuth increases clockwise from north at 0°: east is 90°, south 180°, west 270°. Altitude is 0° at the geometric horizon and 90° at the zenith overhead. A negative altitude puts the object below the horizon.
Hold your view still while Earth turns
Earth rotates from west to east, making distant stars appear to move west. Advance the clock with city and direction fixed and a star's azimuth and altitude change together. Dragging the chart does not advance the clock.
A star's path depends on observing latitude and declination. Some rise and set; others circle a pole without setting at high latitudes. No single arc describes every star.
At the same clock time, Earth reaches another point in its orbit
As Earth travels around the Sun, the nighttime side gradually faces another part of space. At local 21:00 in the same city, the stellar background differs months later. That comparison changes the date, not the length of one night's animation.
Compare January, April, July, and October, recording when a chosen star is above the horizon. The two hemispheres see different sky areas and have opposite seasons, so the controls name months directly.
Catalog positions and local directions use different coordinates
A star catalog records directions on the celestial sphere with right ascension and declination. This lesson reports azimuth and altitude for one city and instant. Change the city or hour and those local numbers change; they are not the star's permanent address.
A sidereal day measures Earth's turn against distant stars and lasts about 23 hours 56 minutes. The civil day follows the Sun's average return and lasts 24 hours, so a star tends to reach nearly the same position roughly four minutes earlier on the next night.
A plotted position is not a visibility forecast
The tool uses a local catalog with a brightness cutoff and coordinates for city centers. Its horizon is a smooth geometric 0°. Hills, buildings, elevation, refraction, and a different observing site can all shift what you actually see.
Daylight and twilight can wash out stars even when they are geometrically above the horizon. This chart does not calculate cloud, moonlight, or light pollution. Planets and the Moon are plotted for the instant you choose, but Uranus and Neptune stay at or beyond naked-eye reach.
FAQ
Does dragging the chart move the stars?
No. Dragging changes your viewing direction. To compare a star's motion relative to the horizon, keep the direction fixed and change the clock.
Why is a star on the chart but not visible outside?
The lesson plots geometric positions. The star may be below the horizon or hidden by daylight, twilight, terrain, or local sky conditions; a dot is not a visibility guarantee.
How do I read azimuth and altitude?
Azimuth runs clockwise from north at 0°: east 90°, south 180°, west 270°. Altitude runs from the horizon at 0° to the zenith at 90°.
This lesson reuses StarWindow's d3-celestial map and Hipparcos catalog. Its coordinate conversion is geometric and does not model refraction, terrain, or sky brightness.