• Choose a month

  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Step 1 – When do we observe?

Knowing when to observe is a real key to learning the night sky – especially in the system of bright stars and asterisms promoted here. And the short answer is to start 30-45 minutes after sunset if you want to learn the bright stars and one hour after sunset if you’re trying to find a new, bright asterism, or refreshing your knowledge of the the night sky to the north.

This entire system is based on a very simple approach: Learn the bright stars and asterisms as they rise in the east. If you start in twilight only these bright stars are visible, so they’re easier to identify. Do this each month and you add to your knowledge. For example, the stars you learn in January will still be in the east in February, but higher in the sky. And they will still be in your March skies as well, but higher still. Each month you want to re-acquaint yourself with the bright stars you learned in the previous month and learn the new bright stars, just rising in the east. By learning them when they rise you will see each new star for several months until it eventually sets in the west. By learning them in twilight first, you will not be confused by the many fainter stars that are near them.

In a similar fashion you should learn to identify the northern pole star, Polaris, and the major asterisms which surround it – then check each month, as it gets really dark, to see how the northern sky has changed in appearance as the stars all appear to rotate around Polaris. Of course, you can do both these things by spending about 15-to-20 minutes under a clear, dark sky once a month – but it’s better to check more often if you can.

You can stop reading right here if all you want to do is focus on learning the bright stars and asterisms. But if you would like to know more about the different stages of twilight and how it impacts what we observe and when, read on!

That said, it does get a little more complicated if you want to understand how to pick the best observing time, for when to observe also depends on what you’re observing. Your decision comes down to three factors – the time of local nightfall, the time of local moonrise, and, of course, weather.

prime_finished_chart

Let’s deal first with nightfall. The chart above uses legal definitions of darkness that depend upon how far the sun is below the local horizon. Civil twilight begins at nightfall and ends when the Sun is 6 degrees below the horizon; nautical twilight begins then and ends when the Sun is 12 degrees below the horizon. That’s when astronomical twilight kicks in. It is not considered to be fully dark until the Sun is 18 degrees below the horizon. This all applies in reverse as we approach daybreak.

These are very precise definitions, but the actual time involved can vary significantly depending on your location and the day of the year. In our chart (and observing) we use them in a loose and general way to represent 30, 60, and 90 minutes after nightfall (sunset). If you want to be precise you can consult an almanac, or if you would like to produce a useful calendar for your location, go to this Web site and follow the simple directions.

I’m not big on precision, however. There are so many things we “know” in astronomy that are really just intelligent guesses and approximations. The important thing to me is understanding what’s really going on, why the sky appears as it does to you at any given moment, and being able to take advantage of this information.

When twilight is prime

Can you spot Venus in this twilight sky? How do you know it's Venus? Because it's the only "star" visible in the East just a half hour before daybreak when this picture was taken. (Venus always appears fairly near the Sun as either an 'evening star' or 'morning star.' )

Can you spot Venus in this twilight sky? How do you know it’s Venus? Because it’s the only “star” visible in the East just a half hour before daybreak when this picture was taken.

One very important example: If you want to learn the bright, signpost stars, start looking when civil twilight ends – about 30 minutes after nightfall. That’s when they’ll first start to appear and when you observe them at that time you won’t be confused by a lot of other dimmer stars because they won’t be visible. This is also the best time to observe the planet Venus in a small telescope. Later, when it’s completely dark, Venus is so brilliant that the glare from it tends to make it difficult to see that it goes through phases much like our Moon – one of the critical discoveries made by Galileo and used to prove that the planets do, in fact, circle the Sun.

And speaking of the moon . . .

Besides the time of nightfall, you should also be aware of when the moon rises and what phase it is in. Whenever the moon is in the sky it hinders the observing of faint objects.

However, don’t give up just because there’s a bright moon, high in the sky when you want to observe. Just adjust your observing goals to it. When the moon washes out a lot of the fainter objects that’s a good time to:

  • Look for your “friendly stars” – since the they’re the brightest stars, they’ll show up and finding them can be easier when there aren’t so many other stars around confusing you.
  • Observe planets and double stars in telescope. Neither depends on a dark sky background.
  • Observe the moon – especially if it is in any phase except full. That’s right – observing the moon when it is full is one of the least interesting times. But don’t worry, we’ll provide plenty of information on moon gazing later.

What about weather?

We can’t see through clouds – but a few clouds won’t ruin an observing session. A high, thin haze will. Fog will. Lots of clouds will. Amateur astronomers are always keeping an eye on the weather, but the typical weather forecast just isn’t geared to the special need of the astronomer, But now there are some special forecasts aimed just at the astronomer. You get these over the web using what is called a Clear Sky Clock and these are scattered all around the country. To find one near you, go here. It’s a tad cumbersome, but I usually focus on the first two rows and I look for dark blue. When you see it, you’ll know what I’m talking about. There’s a new variation, just out, for the entire world, including the US called simply “APanel” or “Astroweather Panel.” You can learn about it by going here.

I have relied mainly on the Clear Sky Clock for the past couple of years and have found it to be amazingly accurate, but do remember it is a forecast and updated every 12 hours.

That said. I actually prefer the display given by the APanel. I just haven’t used it, so I don’t know how reliable it is.

We observe when we’re ready!

But of course, you say, I knew that!  Perhaps – here’s what I mean by “ready.”

First, your mind should be ready. When visitors come to Driftway Observatory we begin with 10 minutes of absolute silence and stillness. If you have meditated, I don’t need to explain why. But there are really two reasons.  The first is to quiet the “monkey brain” – that part of you that keeps jabbering away. (Side trip:To see a good example of the monkey brain go here.) Get the idea? It’s that part of us that wants to multitask – that simply won’t quiet down and focus.  Getting a look at the universe is rare and deserves your attention. It’s a bit like going to church, only in this case you are entering the temple of the universe and it’s nondenominational! So leave all of the  stuff of the day that’s clinging to you behind and focus on the silent messages brought to you from elsewhere in time and space.

The second reason is simpler – give your eyes a chance to dark adapt. Now frankly, this isn’t all that helpful when you are first learning the sky because as your eyes dark adapt you see more and more stars and that can be confusing. Still, it’s a good habit to cultivate because as you learn you’re going to want to see more. The basic idea is simple. In white light our pupils get small and let in less light. In the dark they get large to take in more light. But we can’t control this adjustment – it just happens naturally as it gets dark. For our ancestors living before someone discovered fire it would have been a natural adjustment with the darkening twilight.  Now we step from our well-lit homes or automobiles and into the dark and it takes 10-15 minutes for our eyes to make the major adjustment they need to make. They’ll continue to improve a little for another half hour or so. Unless someone turns on a flashlight, or you look at the headlights of a passing car, or a neighbor’s outdoor security lights suddenly flash on.  These are all things to avoid. Flashlights are handy when you’re trying to find your way in the dark – but let your eyes dark adapt and you’ll be surprised at how much you can see quite naturally even on a moonless night.The night simply is not as dark as you may think.

Astronomers use red light – and as little of it as they can get away with, when they do need light to look  at a sky chart, or maybe adjust a piece of equipment.  This can be achieved by using a rubber band to cover the lens of a flashlight with a couple thicknesses of red cellophane.  There are also several red flashlights available that use a red filter or red LEDs.  They can be found online at merchants selling stuff for amateur astronomers, but I’ve found them in the sporting goods section of major stores as well. The red light doesn’t ruin your night vision, but even this should be used sparingly, especially when you are looking for the faintest objects,  for it does have an impact.

To sum up

  • Most of our observing is in “prime time” – the period that starts about 90 minutes after it gets dark and ends about 90 minutes before day break. However, when we’re trying to find a new, bright star – or some special object near the Sun –  we may observe during twilight.
  • We observe faint objects when the moon isn’t in the sky to interfere, but when it is we stick to bright objects like some double stars, the planets, and, of course, the moon.
  • We observe when it’s clear, though a few clouds won’t ruin the night.
  • We observe when we’ve quieted our monkey brains and let our eyes dark adapt – unless our main goal is to identify a few bright stars.

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