Distance in space
A look at the skies from Sept. 16-22
I was pointing out the star Arcturus (in the constellation of Bootes - the Herdsman) to a friend the other evening, and happened to mention that Arcturus was approximately 37 light years away from us.
She asked what I meant by “light year”. A light year (sometimes abbreviated as “ly”) is an expression of distance (not time) used by astronomers to define how far light, traveling in a vacuum, travels in one year (365.25 days; Julian year), approximately 9.46 trillion kilometres. Thus, the distance to Arcturus is approximately 350 trillion kms.
When we were looking at Arcturus, the light photons (travelling at the speed of light, approximately 300,000 kms/ second) from Arcturus that were hitting our eyes left that star approximately 37 years ago; in a sense, showing us a picture of what the star looked like 37 years ago.
Light years are used to define distances on a very large, galactic scale. For example, Proxima Centauri (the nearest star to Earth other than the sun) is approximately 4.22 lys (39.9 trillion kms) away; the Milky Way Galaxy, of which our solar system is a part, is approximately 100,000 lys (946 quadrillion kms) across; and the Andromeda Galaxy, the only naked-eye galaxy in the northern night sky, is approximately 2.5 million lys from us (I’ll let you do the math on that one!).
Fractions of a light year can be expressed in much smaller units; for example, the “light minute” or the “light second”. Light from the sun takes about 8.2 light minutes to reach the Earth, while light from the moon (actually the sun’s light reflected off the moon’s surface) takes about 1.2 light seconds to reach us.
Distances within our own solar system are usually expressed in terms of “astronomical units” or AUs. One AU is approximately 150 million kms, the average distance of the Earth from the sun. Mercury, our solar system’s innermost planet, orbits at approximately .39 AU (58.5 million kms) from the sun, while Mars, our nearest planetary neighbour, orbits at 1.52 AUs (228 million kms), and Pluto, the furthest planet in our system, orbits at an average distance of approximately 39.5 AUs (5.9 billion kms).
Whether we measure them in terms of light years or astronomical units, the distances within our solar system and to the stars and galaxies beyond, are vast. While we may be able to journey to the planets within our own solar system, given our current limitations with travelling at anything approaching the speed of light, the colourful and exciting voyages to other star systems popularized in science fiction novels and movies are not likely to occur, at least not within this century. But who knows? If we survive long enough, perhaps someday, humans may walk on other planets around distant stars.
LOOKING UP
Jupiter (southwest in Ophiuchus) and Saturn (south-southwest in Sagittarius) remain the only two planets visible in the late evening sky this coming week. There will only be a short window to view either planet once the sky has darkened, before they set; Jupiter around 10 p.m., followed by Saturn around midnight. Viewing the planets will also be difficult, due to their low altitude relative to the horizon. Mercury and Venus are still too close to the sun to be seen. Mars is in solar conjunction (on the opposite side of the sun from the Earth), and won’t be visible again until early November.
The Zodiacal Light is best seen in the weeks preceding and following the equinoxes. With the fall/autumnal equinox a week away, the nights this coming week provide an opportunity, weather permitting, to get a glimpse of this rare celestial phenomenon.
The Zodiacal Light is sunlight reflecting off the countless dust particles floating in the inner portion of our solar system, left there by the myriad numbers of comets that have traversed this area as they swung around the sun over the eons. It is best seen when the ecliptic (the apparent path of the sun, moon and planets across the sky) is tilted at its greatest angle relative to the horizon; above the western horizon just after sunset at the time of the spring/vernal equinox, and above the eastern horizon just before sunrise at the time of the fall/autumnal equinox.
Look towards the east in the pre-dawn hours for a diffuse, pyramid-shaped glow rising up from the horizon to a height halfway to the zenith (point in the sky directly overhead). The farther you are away from city lights, the better the view, especially if there is no light pollution from a town or city east of where you’re standing. It’s a great photo challenge.
For those with a telescope or a decent pair (10x50) of binoculars, you might try looking for Comet 2018 W2 Africano. Discovered in November 2018 by astronomer B.M. Africano, this comet, currently at around mag. 9, is visible this coming week in the constellation of Andromeda - the Princess, almost directly overhead once the sky has darkened. Go online or Google “Comet Africano” to find a current star chart showing the comet’s position in the night sky for the next few weeks.
Until next time, clear skies.
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. His column, Atlantic Skies, appears weekly. He welcomes comments from readers, and anyone who would like to do so is encouraged to email him at glennkroberts@gmail.com.