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Aurora borealis
Photo: Espace pour la vie (Planétarium de Montréal)

Aurora Borealis

What is an aurora borealis?

Polar auroras (borealis or australis, since they also appear in the southern hemisphere) occur when electrically charged particles from the Sun, particularly electrons, hit the Earth’s magnetic field and are guided by field lines to our planet’s polar regions (the Magnetic North Pole is found in the Canadian Arctic islands). In the upper atmosphere (between 100 and 400 kilometres), the electrons strike atoms of rarefied gases (oxygen and nitrogen), which emit light. The atmosphere basically acts as a gigantic neon sign. Auroras are usually greenish, although our eyes can’t detect the colour if the aurora isn’t intense enough. They turn red when the eruption spawning them is particularly intense.

When can we see an aurora borealis?

It’s unfortunately impossible to predict the imminence (and intensity) of an aurora borealis more than 48 to 72 hours in advance. Auroras depend on solar activity. When very large sunspots are visible, violent eruptions can occur. The particles emitted during these eruptions can trigger geomagnetic storms on Earth. An aurora borealis is a visible sign of such a storm. Yet even at the peak of the 11-year cycle of solar activity, there’s no guarantee you’ll see an aurora borealis. Several days and even entire weeks can go by without auroras occurring more intensely than at their basic level.

To track solar and geomagnetic activity and be informed of the high probability of visible auroras, consult daily. The site gives forecasts for the short and mid-term (two to three days) based on what’s happening on the Sun. Also featured are superb photo galleries that many amateur astronomers from Quebec contribute to regularly.

In addition, the Space Environment Center site provides real-time information on solar eruptions (which can cause auroras) and on geomagnetic activity. When the bars in the second graph (at the very bottom of the page) are red and approach the maximum scale value, auroras may be visible even in the city, provided the sky is clear enough.

Here are additional websites measuring the Earth’s geomagnetic activity (which translates into auroras and other events):

Where should we go to see an aurora borealis?

Northern regions are ideal locations to observe this phenomenon given that the frequency of polar auroras increases in higher latitudes. In North America, the auroral oval is a region where auroras are almost daily, albeit usually weak. This band runs across Alaska, Yukon, the Northwest Territories, the areas bordering James Bay and Hudson Bay (Northern Ontario; Abitibi and Baie James in Quebec), and the North Shore. Auroras are fairly frequent (but often near the northern horizon) in the Quebec City region. Its difference in latitude with Montreal considerably improves your chances of seeing one.

Still, when auroras are very strong (following a powerful solar eruption, for example), they may be visible much farther south because the auroral oval widens to cover lower latitudes. On rare occasions, it may extend to the southern United States. In southern Quebec, therefore, visible auroras aren’t that uncommon, provided the weather co-operates. To see them at their finest, it’s best to be outside the city. Auroras are more often visible toward the northern horizon, so if you leave the city, head north to keep the city’s light pollution behind you.


I've heard that we can buy or adopt a star and even name it after someone. Is this true?

Sadly, this business of selling and naming stars does exist.

Yet companies registering stars in your name or someone else’s for a fee and claiming this registration is somehow official are simply after your money. Indeed, anyone can get hold of a database of millions of stars (through the Internet, for example) and start “selling” them one by one. Of course, that means the same stars (especially the few thousand brightest ones visible to the naked eye) could be “sold” by different companies (or even the same company) to different people.

To put it bluntly, if you bought a star from these companies, you could never point to the sky and say, “That one’s mine.” You’d need at least binoculars and probably a telescope to locate and single out “your” star from the thousands of other points of light, a task even seasoned amateur astronomers might find difficult.

The Montreal Planetarium opposes and discourages the selling and naming of stars. We endorse and follow the recommendations of the International Astronomical Union, which has posted an interesting FAQ on this topic on its web site.

If you’re still bent on “buying” a star, you’ll find more than enough information on the Web (companies in this business are easy to find). But we urge you to remember that stars are not for sale.

Finally, some serious and renowned institutions have been “selling” stars for fundraising purposes. But they clearly state that the sale is symbolic and has no legal or scientific meaning. For instance, the H.R. MacMillan Space Centre in Vancouver has set up such a program.

I’ve noticed a star whose sparkle is extremely strong and seems to contain glints of red and blue. Why is it twinkling so brightly?

A star’s twinkle and rapid change in colour are due to atmospheric turbulence. The mixing of pockets of hot and colder air splits light like a prism, resulting in the changing colours you see. The effect is stronger on cold, clear nights and more noticeable with brighter stars, especially when they aren’t very high in the sky.

Sirius, the brightest star in our sky (visible especially in winter and spring), emits an intense bluish white light. Since it never rises far above the southern horizon, Sirius is particularly subject to this phenomenon.

Antares in the constellation Scorpius (especially visible in summer) is another star that remains close to the horizon. Though reddish in colour, this star sometimes gives off remarkable glints of green.

Other bright stars may also produce a similar effect. Consult the Monthly Sky page to identify the stars and constellations visible at this time of the year.

The Moon

I’m looking for the dates of the phases of the Moon. Where can I find them?

Our website contains a lunar phase table from 2001 to 2006. Outside this period, you can query the ephemerides server of the U.S. Naval Observatory in Washington (for an entire year, from 1700 to 2035) or the Institut de mécanique céleste et de calcul des éphémérides in Paris (one month at a time, from 4000 BC to the year 2500). Realize, though, that these servers give the date and hour in universal time (UT). To get Eastern Standard Time (EST), you must subtract five hours, thereby possibly changing the date. For example, January 4 at 3:12 UT equals January 3 at 22:12 EST.

Last night, I spotted a bright “star” near the Moon. But tonight, it’s no longer there. Why?

With respect to the stars, the Moon travels fairly quickly in the sky. Indeed, it moves around the celestial sphere and returns more or less to its initial position among the constellations in about 27.3 days. From midnight one night to midnight the next, the Moon moves about 12 degrees farther in the sky. Each month, the Moon moves close to naked eye planets and to certain bright stars. The planet or star you noticed near the Moon one night will be approximately in the same spot in the sky the next night, whereas the Moon will have moved eastwards (to the left).


For a few days now, I’ve noticed a very bright “star” at dawn or dusk. Might this be the International Space Station?

Chances are that your star is in fact a bright planet such as Venus or Jupiter. Consult Astronomical Events on our website to find out which planets are visible at the moment.

The space station and all the artificial satellites visible to the naked eye show obvious motion. In fact, they cross the sky in minutes. (Their movement is like a high-altitude airplane’s, although without the blinking lights, navigation lights or engine noise.) You’ll spot them especially in the early evening or at the end of the night when the sky in your area is fairly dark and the satellites, a few hundred kilometres high, are still lit by the Sun.

Some websites predict the time that the major satellites pass overhead and the direction you can see them in from your specific location. See the Artificial Satellites section on our Astronomy Resources on the Web page.

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