Lesson 6 of 7 · Reading a sky chart
Turn a chart into a direction to look
8 minute lesson · 3 checks along the way
Your goal for this lessonMatch an observer’s location and time to a chart and interpret altitude, azimuth, horizon, and projection labels.
Words with a dotted underline open a short definition right here in the lesson.
Set the observer before finding the object
A sky chart is a prediction or representation for a particular viewpoint. Before searching its dots, check the or location, date, time, and time-zone setting. A chart for northern middle latitudes is not automatically a chart for southern middle latitudes. Even at one location, changing the time moves much of the predicted sky relative to the . A correct label cannot rescue incorrect observer settings.
Many circular whole-sky charts place the , directly overhead, at the center and the horizon around the edge. This is a view looking outward toward the sky, not a ground map looking down at Earth. East may therefore appear on the opposite side from its place on a conventional north-up road map. Read the directions printed on the particular chart instead of assuming an orientation.
Use two angles with declared conventions
here means angle above the local horizon: 0° at the horizon and 90° at the zenith. It is not height in kilometers. specifies direction around the horizon. In this lesson it is measured from true north through east: north 0°, east 90°, south 180°, west 270°. Other conventions exist, so labels and documentation matter whenever you change tools.
An object at altitude 30° and azimuth 90° is above the eastern horizon, one third of the angular way from horizon to zenith. Its actual distance remains unknown from these two coordinates alone. A nearby aircraft and a distant star could briefly share nearly the same direction. The chart tells you where to point your attention, not how far the object lies along that line.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Select an observable training target
The original instructional chart contains fictional targets, not tonight’s real stars. A is at altitude 30°, azimuth 90°; B is at altitude 60°, azimuth 210°; C is at altitude −10°. A lies east, B lies toward the southwest and higher in the sky, and C is below the geometric horizon. If a building blocks the eastern sky up to 35°, it also hides A.
B is the better candidate under those stated conditions, provided that its southwest direction is clear and the sky is dark. The chart alone does not know about the building. The 30° and 60° altitude rings describe angular positions, and the plotted dot sizes distinguish labels without specifying stellar diameters. A below-horizon target cannot be recovered by increasing the screen brightness or rotating the page.
Choose a target above the obstruction
Original instructional chart, not tonight’s sky. It looks outward with north at the top and east at the left. The center is the zenith, the rim the horizon, and rings mark angular altitude.
Open full-size diagram in a new tab: Choose a target above the obstructionCheck 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Match the chart to a small real pattern
At a permitted nighttime location, face the direction marked for the chart’s selected edge and orient the chart according to its instructions. Begin with a bright object and two nearby stars rather than trying to recognize the whole sky at once. If using an app, verify its clock, location, and orientation against known landmarks; sensor pointing is a convenience, not independent confirmation of an identification.
Polaris offers an approximate northern reference where it is visible: its altitude is close to the observer’s north latitude because it lies near the north . It is not the brightest star and is not a universal guide for southern observers. Whether using Polaris or local landmarks, keep direction, horizon obstructions, and the chart’s perspective explicit. Successful chart reading is a match between a model and an actual view.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.
- Read: U.S. Naval Observatory: altitude and azimuthObserver-dependent horizon coordinates and the north-through-east convention.
- Read: NASA: skywatching FAQCharts, local settings, hemisphere limits, and Polaris.
- Read: NASA: reference systemsPolaris altitude approximates northern latitude; this is an angular reference.