Using a planisphere during a clear night session. Image: Wikimedia Commons (CC BY-SA 4.0)
Star charts and planispheres are among the oldest tools in astronomy. Before smartphones and planetarium software, observers navigated the sky with printed maps and a sense of direction. These paper tools remain relevant: they require no batteries, work in any weather (within reason), and force a more deliberate, tactile engagement with the sky.
What Is a Planisphere?
A planisphere is a rotating circular chart consisting of two parts: a star disc showing all stars visible from a given latitude, and an oval window cut in a horizon ring that reveals only the portion of sky above the horizon at a specific date and time. By rotating the disc until the current date aligns with the current hour on the outer ring, you see a map of the sky as it appears from your location at that moment.
Planispheres are latitude-specific. A chart designed for 43°N (roughly the latitude of Toronto) will be noticeably inaccurate if used in Edmonton (53°N) because the altitude of the celestial pole differs by 10 degrees, shifting which stars rise, which are circumpolar, and how high constellations appear above the horizon. Choosing the correct latitude for your location is the most important factor when selecting a planisphere.
Latitude Reference Points in Canada
Toronto: 43°N • Ottawa: 45°N • Montreal: 45°N • Calgary: 51°N • Edmonton: 53°N • Vancouver: 49°N • Winnipeg: 50°N • Halifax: 45°N
Celestial Coordinates
The night sky uses a coordinate system analogous to latitude and longitude on Earth. The two values are right ascension (RA) and declination (Dec).
Right Ascension
Right ascension measures position east-west along the celestial equator. Rather than degrees, it is measured in hours, minutes, and seconds — a convention inherited from the way the sky appears to rotate overhead as Earth turns. One hour of RA corresponds to 15 degrees of arc. The zero-point (0h) is the vernal equinox, the position where the Sun crosses the celestial equator each March.
As an example, the Orion Nebula (M42) is located at approximately RA 5h 35m, Dec –5°. This places it just south of the celestial equator, meaning it is visible from all Canadian locations (though it remains relatively low in southern Ontario winter skies compared to locations at the equator).
Declination
Declination is equivalent to latitude on Earth and is measured in degrees north (+) or south (–) of the celestial equator. Objects with high positive declination (such as the Andromeda Galaxy at Dec +41°) remain above the horizon for a large portion of the night from Canadian latitudes. Objects with strongly negative declination, such as the Southern Cross (Dec –60°), never rise above the horizon from any Canadian location.
Orienting a Star Atlas
Most printed star atlases present individual chart pages with north at the top and east to the left — the opposite of a conventional map. This is because you hold a star chart overhead, facing the sky, which reverses left and right relative to a map viewed flat on a table.
When orienting a chart in the field:
- Identify north using a compass or by locating Polaris (the North Star), which sits within one degree of the north celestial pole.
- Face north and hold the chart above your head, with the north edge of the chart toward north.
- East on the chart will now align with actual east, and west with actual west.
- To look south, turn around and rotate the chart so the south edge points toward south.
Finding Polaris
Polaris is the single most useful navigational star for observers in Canada. It sits close enough to the north celestial pole that it appears stationary while all other stars trace arcs around it over the course of a night. Finding Polaris takes about 30 seconds once you know the method:
- Locate the Big Dipper (Ursa Major). In Canada it is circumpolar — always above the horizon, though its position rotates through the night and seasons.
- Find the two stars at the outer edge of the Dipper's bowl (Dubhe and Merak). These are called the Pointer Stars.
- Trace a line from Merak through Dubhe and extend it roughly five times the distance between them. The moderately bright star at that position is Polaris.
Polaris is not especially bright — it ranks around magnitude 2.0 — but it is the brightest star in that region of sky, making it identifiable once you know where to look.
Seasonal Skies from Canada
The portion of sky visible changes through the year as Earth orbits the Sun, bringing different constellations into the evening sky each season.
Seasonal Highlights (Evening Sky)
Winter (Dec–Feb): Orion, Taurus, Gemini, Auriga. Best time to observe the Orion Nebula, Pleiades, and Hyades cluster.
Spring (Mar–May): Leo, Virgo, Boötes. The Virgo Galaxy Cluster (around 54 Messier objects) is accessible from dark sites.
Summer (Jun–Aug): Scorpius low in the south, Sagittarius, Cygnus, Lyra, Aquila (the Summer Triangle). The Milky Way core is best placed.
Autumn (Sep–Nov): Pegasus, Andromeda, Perseus. The Andromeda Galaxy (M31) is well positioned for observation.
Useful Resources for Chart Selection
The Royal Astronomical Society of Canada publishes an annual Observer's Handbook — a widely used reference that includes monthly sky charts calibrated to Canadian latitudes. The handbook is available through RASC and many public libraries.
For those who prefer digital tools alongside paper charts, Stellarium (free, open-source) produces accurate sky simulations for any date, time, and location, and can be used offline.