You may not realize it, but every time you gaze at the night sky, you’re taking a dizzying journey through space and time. September is the ideal month to embark on this cosmic adventure!
As mere humans relying only on our senses, it’s impossible for us to judge the distance of the stars: The sky appears two-dimensional to us. But, as you can imagine, the sky isn’t a stationary dome hanging over our heads, it’s a three-dimensional space.
Our gut may tell us that the brightest stars are the ones closest to us, and that’s generally true. But things aren’t always as they seem.
Take the famous Summer Triangle, for example, clearly visible this month directly overhead as darkness falls (check out the map).
Vega (top right corner of the triangle) is 25 light-years away from us, while Altair (bottom corner) is 17 light-years away. These two stars are relatively close, astronomically speaking. Now find Deneb, in the top left corner: It lies at an astounding distance of 1,550 light-years, nearly ten times farther away!
That means the Summer Triangle is actually quite distorted in space: Its top-left corner is much more distant than the other two.
Another example is Mu Cephei, the hypergiant star in the constellation Cepheus, also known as the Garnet Star because of its deep red colour. With a diameter 1,300 times that of the Sun, it is 100,000 times more luminous than our local star. But Mu Cephei doesn’t appear that bright just looking at it (magnitude +3.6), since it could be up to 5,300 light-years away. Its exact distance is difficult to determine, but it’s one of the most distant stars visible to the naked eye.
In the countryside or a local park, picture how far your gaze extends as you peer into the night sky. Imagine beams of different lengths—some short, some long—reaching out from you to each individual star, over distances spanning trillions of kilometres. Your mind will be boggled!
But the mind-boggling doesn’t stop there... Light travels at 300,000 km per second. In one year, it travels a distance of one light-year, or about 10 trillion kilometres. And that’s why it takes a long time for starlight to reach us: exactly one year for each light-year. When we say that Vega is located 25 light-years away, that means it takes 25 years for its light to get here. In other words, the light we see today began its journey 25 years ago. Looking at the Summer Triangle is like being whisked 25 years, 17 years and 1,550 years back in time. And when we gaze up at Mu Cephei? That’s like travelling 5,300 years into the past. Wow!
Also observable in September is the Andromeda Galaxy, one of the most distant objects visible to the naked eye. When looking at it with our own eyes or binoculars, our retina captures light that has been travelling for 2.6 million years!
Thanks to the power of both astronomy and the imagination, we can travel through space and time simply by observing the sky.
The Solar System
Closer to home, there aren’t many remarkable phenomena visible in the September sky this year. The full Moon occurs on the 14th at 12:33 a.m. EDT and the new Moon is slated for the 28th at 2:26 p.m. The equinox, which marks the beginning of astronomical autumn, happens on September 23 at 3:50 a.m.
As for the planets, only Jupiter and Saturn are visible, relatively low on the horizon at nightfall. The very brilliant Jupiter is the first to appear at twilight, in the south-southwest. Saturn pops into view as the sky grows darker, culminating in the south, 22 degrees above the horizon. On the evening of September 5, at twilight, the first quarter Moon lies a few degrees to the right of Jupiter. The following evening, the gibbous Moon swings to the left of Jupiter and on September 7, it shifts to the right of Saturn. Finally, on the evening of September 8, the Moon can be found 5½ degrees to the left of Saturn at twilight.