AstroWeek Part 1: Compass & Clock

Alright, I’m doing more sky-stuff this week. Not just oh-hey- here’s-a-planet-type posts, but some useful, big-picture, stuff-you-can-use-every-night posts. This week is AstroWeek at WTWWU, and it’s gonna be awesome.

Tangent: I love doing “theme weeks.” It makes me feel like I’m going somewhere and not just screwing around*. The first one I did- that I think is still my favorite- was that Dandelion Week. Bird Feeder Week kind of kicked ass too. And Monocot Week- even though only 2 of the 4 posts ended up actually being about Monocots- was fun because it was full of road-tripping (mis)adventure.

*Yes, I know deep down I’m still screwing around. But every middle-aged man needs a hobby, and this one is way better than gambler, stock-car racer, philanderer or serial killer.

Preamble Explaining Why You Should Pay Attention To This Post

Here’s why it’s good to know something about the night sky: For months and months throughout the Summer, you hardly ever see darkness. By the time it gets really dark enough to see the stars, you’re about ready to go to sleep, and when you wake up it’s already half-light. But after the Fall Equinox all that changes. It’s dark when you wake up and then again long before you go sleep and pretty soon it’ll be dark when you leave work to drive home.

For outdoorheads, the darkness presents a choice: withdraw, limiting one’s activities to either weekends, lunchbreaks or indoors (gym, trainer, etc.) and mentally set one’s sights on Spring, or deal with the darkness. As longtime readers know, I’m a deal-with-it guy, biking, skiing and running in (usually pre-dawn) darkness. But over the years of doing stuff in the dark, I’ve become convinced that there are actually 2 ways of dealing with the dark.

The first is to steel oneself with lights, warm clothing and a tough attitude, a determination to bike/run/ski despite the darkness, toughing it out against the enveloping blackness. Though character-building, this “opposition” approach sees night as the enemy, something to be overcome and endured.

But the second is to understand, anticipate and even welcome nightfall, to feel comfortable and at ease in the backcountry at night, even when alone. And the best way to feel at ease outdoors at night is to know the night sky. While lights and warm clothing are still necessary, the attitude really isn’t; when you know the night sky, you always know where you’re headed and what time of “day” it is. 10PM and 6AM may still both be pitch dark, but they look completely different in the sky above, and once you recognize the difference 6AM changes from dark and scary to early and cheerful.

The good news is that to orient and tell time at night you don’t need to be some kind of star-nerd; you need to know just 2, super-simple constellations. A third, or even more will fill out the picture, but just 2 will do it, and you can see them both all night long.

The Neighborhood

But first, a little orientation. When you look up at the sky at night, what are you looking at? If your only answer is “stars”, that’s a little lame. It’s like walking around in Salt Lake and someone asks you, “What are those high things to the East?” If you just say “mountains”, that doesn’t really tell them anything. But if you were to tell them, “The Wasatch Range”, or “The Westernmost range of the Rockies”, or the “The Eastern edge of the Great Basin” or the “The range raised by the Wasatch Fault”, any one of those answers tells the asker way more about where they are, what they’re looking at and its significance. So let’s make some sense of the night sky.

Our sun is one of 200 billion+ stars in the Milky Way galaxy. The Milky Way is a spiral galaxy, which means it’s a flattish disk with a bulging center, and multiple spinning, trailing spiral arms around it. The whole thing is spinning round and round, and in fact our sun orbits the galactic core roughly every 220 million years*. Outside of the core the stars are largely grouped in these arms, with mostly empty space in between.

*One “galactic year” ago the world was still largely covered in forests of lycophytes and Tree Ferns, and early dinosaurs were walking around. Two years ago the first plants were venturing on land, but nothing like a tree or a land animal yet existed.

The Milky Way has 4 primary arms, from innermost (Norma) to outermost (Perseus) and numerous lesser, “minor” arms in between. Our sun lies in the Orion Arm, a minor arm in between the 3^rd (Sagitarius) and 4^th (Perseus) arms, which is 3,500 light years across and 10,000 light years long. When we look up at night, the majority of stars we see in the sky are in the Orion Arm.

Side Note: Not all, though. For example Eta Leonis, the star that connects Leo’s front legs to its neck, lies ~2000 light years away in the Perseus Arm.

The first, easiest thing to find in the sky is the Big Dipper. It’s always in the Northern Half of the sky, and after sunset right now (mid-October) it’s low in the sky and right-side up. It’s actually part of a bigger larger, “official” constellation called Ursa Major, the Big Bear, but the dipper- a saucepan actually- is way more obvious. 4 stars make the pan and 3 the handle.

Tangent: There are 88 official constellations, but only 1 other (Orion) is as flamingly-obvious as the Big Dipper. Many others are obvious and easy to recognize, but not as what they’re supposed to be. Cassiopeia for instance is super-easy to find, but it doesn’t look anything like the mother of the maiden chained to a rock as a sacrifice to a sea-monster but subsequently rescued by Perseus. No, it looks like the letter “W”, and I don’t know why they just don’t call it “W”.

This is probably my biggest beef with constellations, and what makes them so darn frustrating for “regular” people: they don’t look anything like what they’re supposed to look like. You’ll be out on a beautiful night with some star-nerd and they’ll point out some obscure constellation, saying, ”Yes, that’s Pegalophorus, the young boy riding a 3-legged pony while playing the lyre…” and you’ll be like, “Where?” and after maybe 20 minutes of pointing stuff out you’ll realize that the whole constellation consists of 3 stars. A lot of these constellations really seem to be somewhat of a stretch, and I can only assume that, lacking television and HID lights, the ancients had a lot of spare time and imagination when staring at the night sky.

Ursa Major, the “official” Big Dipper constellation, is another example: It’s a bear… with a tail. What bear has a tail?

Look at the saucepan, specifically at the 2 stars which comprise the non-handle side of the pan. Follow the line they make “up” from the pan to the next bright star. This is Polaris, the North Star, which is within 1 degree of true North. We’ll come back to Polaris in a moment, but for now, continue on a bit further past it.

On the other side of Polaris, at roughly the same distance from it as the Big Dipper, is a big “W”. At dusk in October the W is on its side, and whatever time of night it is, it will always be positioned such that Polaris is directly “above” the W if the W were right-side up. The W isn’t directly across Polaris from the Dipper; if Polaris were the center of a (12 hour) clock, the W would be about 7 “hours” ahead. The W is Cassiopeia.

Compass

OK, got it? If you found the saucepan and the W, the night sky is now your compass and your clock. The compass part is easy- the saucepan and the W are always roughly across from each other, and have the North star between them, and “above” each of them (if they were right-side up.) There are other easy navigational markers in the sky, one of which I’ll get to in a moment, but for now, that’s all you need. If the sky is clear, North is obvious and accurate, easier to find than with a magnetic compass or a GPS.

Side Note: Why easier than a compass? Because to use a compass you need to know the local magnetic declination, or difference between True North and magnetic North. The Magnetic North Pole isn’t at the geographic North Pole; it’s up in the Canadian Arctic, and so here in Salt Lake there’s a 12 degree difference between True North and Magnetic North. It gets worse- the Magnetic North Pole is on the move. Fortunately for compass-users, it’s been getting more accurate over the last several decades, as the Magnetic Pole has gotten closer to the True North Pole. When I was born (1964) the magnetic declination in Salt Lake was more like 16 degrees.

Wait- it gets even worse. Magnetic compasses are also influenced by local magnetic fields induced by things like wristwatches or the boat or plane you’re travelling in. The net-net? Most of us will never get a magnetic compass to perform within a degree of True North, which is what Polaris gives us.

It’s of course a lucky coincidence that Polaris is straight “up” relative to the North Pole. Polaris is a fascinating star. It’s 420 light years away and is huge. It’s 2500 times more luminous than the sun and way, way bigger; it’d be as if the sun’s radius extended out to the asteroid belt (between Mars & Jupiter.) It’s also a multiple star, specifically a triple star.

Side Note: Let’s clarify “up.” Polaris is “up” relative to the Earth’s equatorial plane. But the Earth- and it’s equator- are tilted 23 degrees relative to the solar orbital plane- called the ecliptic. But the ecliptic is tilted 60 degrees relative to the galactic plane, which explains why you see the “Milky Way”- the galactic core, 26,000 light-years away- spread across the sky from Cassiopeia to Sagittarius, and not down South by the Moon and the planets. Thinking too much about “up” in space will drive you batty*.

*I started to do a 3-D graphic showing the 3 planes in question, but it drove me, yes, batty.

Our sun is unusual in that it’s a solitary star; most stars in the universe are multiple stars- binary, triple, or more (as we’ll see in tomorrow’s post.) Polaris has 2 companions, neither of which is large enough to see. Both are much smaller, “dwarf” stars. The first orbits Polaris every 30 years in a wildly eccentric orbit. The second lies way farther out- almost 50 times as far as Pluto is from our sun, and orbits the primary star every 42,000 years.

Even cooler, Polaris prime is an older star, and its core is “burned out.” It’s done fusing hydrogen in its core (which is how “healthy” stars- like our sun- shine) and its structure is now supported via helium fusion in the core. But the structure is unstable, and so the star pulsates, varying in brightness. Every 4 days Polaris goes through a brighter-dimmer cycle. The difference is too small for us to notice, but astronomers can measure it. This type of star is called a Cepheid variable. Cepheids are old, unstable stars, wheezing along through their golden years, as it were.

But the most interesting thing about Polaris is that it wasn’t always- and won’t always be- the North Star. Part of the reason is the long-term motion of stars in the galaxy, but the more pressing reason is that the Earth wobbles. Every 26,000 years the Earth’s axis spins around on a virtual cone- like an unstable spinning top- in a process called Precession of the Equinoxes (diagram right, not mine). Right now Polaris is “getting” closer to True North, and will continue to do so until 2105, whereupon it will start drifting away relative to an Earth-bound observer. Over time other stars will become better indicators of True North and by the year 20,500AD one of the stars in the Big Dipper will be the “North Star”, as we’ll see in tomorrow’s post.

Clock

OK, so we know where we’re headed. But even cooler, as the night progresses, the entire sky revolves around the North Star, and if we learn to associate the relative positions of the Big Dipper and Cassiopeia with rough times, we can use the night sky as a clock. There are 3 big caveats:

1- The clock’s positions change a little each month throughout the year. If you’re familiar with the positions of Big Dipper and Cassiopeia around sunset and sunrise in October, but then ignore the sky for several months and then pop your head out one night in February, everything will be different.

2- It’s a 24-hour clock, not a 12-hour clock. You see 24 clocks often in places like European train stations, but they’re pretty unusual here in the US.

3- The clock runs counter-clockwise. This makes sense- the Earth rotates to the East, making the sky roll into the West- but it’s the most counter-intuitive part of the whole sky-clock.

Right now (mid-October) when it gets dark here in Salt Lake (~8PM MDT), the Dipper is low in the sky and right-side up, at about 7 o’clock (if the sky were a 12 hour clock). Cassiopeia is at roughly 2 o’clock, “W” on its side, tilted about 120 degrees to the left.

When I pedal out of my driveway at 6AM, the Dipper is at 3 o’clock, and Cassiopeia at 10 o’clock. When I see them in these positions, I know it’s “morning” and that daylight is only an hour away.

You can easily extrapolate between these times. For example if you’re camping and you wake up in the middle of the night and wonder what time it is and you look up and the W is completely upside-down, above the North Star, you know that it’s like 1 or 2 in the morning (in October) and you should roll over and go back to sleep. But if the W’s tilted on its right side relative to Polaris, you know dawn is just around the corner, and you might as well get up and make coffee.

You can also tell time- and direction- by looking at the Southern sky, but the Southern sky doesn’t work like a clock so much as it does a procession of constellations progressing from left (East) to right (West) through the night. I’ll highlight just one, because it’s super-easy, super-spectacular, and right now it’s straight up in the sky just before dawn- Orion. It’s the hunter with the bow, and I’ll cover it later in the week when we talk about nebulae, red giants and supernovae. But from a clock perspective, right now seeing the hunter up in the sky means it’s morning. Daylight is an hour away; you should get up and get outside.

There’s something else easy to see right by Orion that’s worth checking out. Below and to the left/East of Orion is a really bright star. It’s Sirius, the brightest star in Canis Major (big dog.) Sirius is unusual and worth knowing for 2 reasons. First, it is the brightest star in the sky. Second, it’s one of the closest, at just 8.6 light years away*, or roughly 50 times closer to us than Polaris.

*The closest star, Alpha Centauri (a triple star system) is about 4.5 light years away.

This is a good time to talk about distance. If you shrank the whole universe so that the sun was the size of a basketball, and you placed that basketball-sun in Salt Lake City, how far away do you think Sirius would be? Take a moment... Provo? Nephi? Moab? Denver?

Give up? Sri Lanka. That’s right, Sirius would be over 11,000 miles away*. Think about that. On a flat plane you can’t even see a basketball 1 mile away**, but stars are so bright they’re like seeing basketballs*** on Sri Lanka. Seeing Polaris on this scale is like seeing a medicine ball on the moon****.

*My calculation, could be wrong.

**Astute readers might wonder if one could see- even with supervision- a basketball a mile away on a flat plain, due to the curvature of the Earth. The answer is yes, mainly because you’re standing up a few feet above the Earth. But even if you lay down on your belly and got your eyeball down at ground-level, the mile’s worth of curvature would conceal only the bottom 8 inches of the basketball from your view.

***More like a beach ball in this case, since Sirius is about 1.75 times the diameter of the sun.

****This one’s a bit off. More accurate would be a VW bug at the twice the distance of the moon, but I wanted to pick something on the moon. OK, 4 footnotes? I need to cut this out.

Even if you don’t know anything about the stars, this, the sheer scale of the things we can see, makes the night sky the most awesome sight around. Sirius, BTW is a binary star, with a small (about the size of Earth!), dim, hot companion star. The 2 orbit each other every 50 years at distance equivalent to that between the Sun and Uranus. Sirius A- the star we see- is way, way brighter, but Sirius B is actually hotter. Anyway, in mid October, when you see Sirius nipping at Orion’s foot, it’s time to ride.

I know, this all sounds pointless- why not just look at your watch? You can and you should, but my point is this- if you intuitively recognize the time of night in the sky, then night stops being scary. And when night stops being scary, it’s no longer time taken away from you. You get it back.

Next Up: The Big Dipper is way cooler than you ever knew.

UPDATE 9:30AM: After posting this morning I drove up to Jeremy Ranch to ride 24/7 and Flying Dog. When I got out of the car it was pitch dark, freezing cold, and I glanced up at the “clock” for reassurance. As I did so, that very second, a slow, long-lasting (close to a full second) spectacular meteor streaked right across the face of the clock. Cool way to start the week.