David Dickinson, Earth science teacher and backyard astronomer, and Fraser Cain, publisher of Universe Today, have teamed up to provide expert guidance on observing the night sky.
The Universe Today Ultimate Guide to Viewing the Cosmos features the best tips and tricks for viewing our solar system and deep sky objects, as well as detailed charts, graphs and tables to find must-see events for years to come. This comprehensive guide is complete with stunning and exclusive photography from top night sky photographers, as well as advice on how to take your own incredible photos.
Take your recreational viewing to the next level with activities like:
Finding comets and asteroids
Tracking variable stars
Monitoring meteor showers
Following solar activity
Timing lunar and asteroid occultations
With star charts, practical background information, technological resources and telescope and astrophotography guides, this is the ultimate resource for any backyard space enthusiast.
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About the Author
Fraser Cain is the publisher of Universe Today. He’s also the co-host of Astronomy Cast with Dr. Pamela Gay. He lives in Courtenay, British Columbia.
Read an Excerpt
STEPPING OUT TONIGHT
Finding your way in the dark and knowing how to speak astronomer.
"I have loved the stars too fondly to be fearful of the night."
— Sarah Williams
PROTECT YOUR NIGHT VISION.
You're going to need it.
Before you read any further into this guide, here's a challenge: Right around sunset tonight (assuming the skies are reasonably clear), simply step outside and check out the night sky.
What would you see, gazing into the sky?
Once the Sun sets, you enter the period known as dusk, a twilight realm where the lighting changes almost moment to moment. Photographers are familiar with this special time. Day shifts to night as the Golden Hour gives way to the Blue Hour. Local objects are drained of their color, as darkness deepens and the cone cells of our eyes switch over to their more sensitive rod cells in the dwindling light, like switching from fast to slow film in daytime versus nighttime shooting. Using an eye patch or sitting in a darkened room can even allow you to dark adapt in advance for an upcoming session of nighttime observing. The better your eyes are dark adapted, the more you'll see. The bright screens of modern electronics can ruin night vision, and many amateur astronomers will cover them with either a red-light filter to preserve night vision, or set apps to adjust the light to a dimmer, reddened night-vision mode while observing. Several apps are even available to automatically dim screens and filter out blue light during twilight hours.
Protect your night vision. You're going to need it.
What do you see tonight? Is the Moon visible? The twilight period during dawn or dusk is actually divided off into three types: civil, nautical, and the darkest of all, astronomical twilight. Perhaps, as twilight darkens, you might spy a star moving slowly across the sky. This is a satellite, a product of the modern Space Age. Satellites, for the most part, move across the sky with the rotation of the Earth from west to east, unwavering moving "stars" illuminated by the Sun in the twilight that soon wink out upon hitting the Earth's shadow. Most of these are old rocket boosters, silently tumbling end over end in orbit. Every week or so, you might just spy the International Space Station. Humans have continued to occupy this orbital outpost in space since the arrival of Expedition 1 in 2000.
Watch the sky long enough, and even more curious sights can appear.
Living at middle and high latitudes, you might just see the flicker of auroras, the interaction of the Earth's upper atmosphere with the solar wind, proving that we do indeed live inside the atmosphere of our host star, the Sun.
A meteor may punctuate the night, a flashing emissary of a comet whose path intersected that of the Earth long ago. These pick up in tempo during annual meteor showers, and very occasionally, put on one of the most stupendous displays in astronomy during great meteor storms.
Then, one by one, the stars appear. They're there in the daytime as well, when they are overpowered by the brilliance of the Sun. These same patterns visible tonight remain nearly unchanged through historical times, a nighttime backdrop for human drama on which we pin our hopes and fears. The stars flicker as their light — traversing the vacuum of space for centuries — is distorted in its final fraction of a second journey through our atmosphere.
HERE'S A HANDY GENERAL RULE:
Stars flicker or twinkle; planets don't.
You might also notice a few beacons of light, unwavering in their constancy. These are planets, whose tiny disks are still just large enough to remain less affected by the roiling convection cells in our Earth's atmosphere.
Here's a handy general rule: Stars flicker or twinkle; planets don't.
MEASURING THE SKY
HOW CAN WE DESCRIBE THE POSITIONS OF OBJECTS IN THE SKY?
How can we describe the positions of objects in the sky? The illusion of the sky as an upside-down bowl overhead covering our little patch of land out to the local horizon is a handy one to understand the motion and position of things overhead. Celestial cartographers used the analogy of a geocentric Earth inside a crystalline, transparent sphere covering the vault of the sky, and even constructed models of this early structure of the Universe as seen from the outside vantage point in space, looking in.
Today, we know this is a fantasy and an illusion, owing to the enormity of the Earth and the Solar System versus the tiny patch of our local vantage point out to the local horizon a few miles away. For example, the curve of the Earth is immediately apparent on the face of the Moon during a lunar eclipse. This curve can be seen at any angle, whether the eclipsed Moon is low to the horizon or high overhead.
Think of the true horizon as an imaginary line where the Earth meets the sky, running a complete 360-degree circle around tonight's observing patch. Looking out along the ocean from the beach is the closest to the true horizon that you can ever see, a line straighter than the hand can draw. Of course, this line is indeed curved a tiny amount, slighter than the eye can discern. Earth is just that big.
The point directly overhead is the zenith, and the imaginary opposite point at your feet is the nadir. On Earth, the nadir point is forever invisible, though in space, you can see objects both at the zenith and nadir.
We use degrees of arc to measure the distance between one object and another in the sky. This separation is apparent — as in one object may be much closer or farther away than the other — and biased as seen from our Earthly vantage point. The circumference of the sky can be further divided up into 360 degrees of arc, with each degree divided into 60 arcminutes and each arcminute into 60 arcseconds.
The zenith, for example, is located 90 degrees above the local horizon, straight overhead; the Full Moon is about half a degree (30 arcminutes) across, meaning you could actually stack 180 Full Moons from the horizon to the zenith.
You can make rough measurements of objects in the sky simply by placing your open hand at arm's length. (See this chapter's activity, Measuring the Sky, here.) The span of your hand is about 10 degrees across, and each finger is 2 degrees across. Try it next Full Moon: you can actually hide four Full Moon widths behind your thumbnail at arm's length!
Smaller measurements such as arcseconds only come into play when we're talking about views under magnification, such as the separation of close double stars or the apparent diameters of planets. One arcsecond is 1/3600 of a degree, a smaller angle than the human eye can discern. The smallest angle in the sky an eagle-eyed observer might split is just under an arcminute. Likewise, double stars under an arcsecond apart represent a difficult split at the eyepiece under magnification. Professionals further divide arcseconds into milli- (thousandths) and micro- (millionths) of an arcsecond; truly tiny angles, indeed. These minuscule angles only come into play when astronomers measure parallax — the tiny shift in a star's apparent position as a function of its true distance — and occasionally, the angular diameters of the very largest stars themselves.
Exploring the upside-down, bowl-shaped illusion of the sky overhead, we can then describe the position of an object using a two-coordinate system, with azimuth as the left-right position on the horizon starting at zero (north), to 90 degrees (east), 180 degrees (south), and 270 degrees (west), and the altitude or elevation as the object's position above the horizon from zero (on the horizon) to 90 degrees (at the zenith).
ACTIVITY: HOW FAR CAN YOU SEE? A VISUAL TOUR OF TONIGHT'S SKY
As mentioned previously, the night sky looks like someone simply overturned a bowl above our observing site. Of course, this is an illusion, and things overhead in the sky are (relatively) near or far (very far) away.
But how far can you see? It's a question often asked at star parties. Let's look at the night sky, with distance in mind.
Space begins about 62 miles (100 km) overhead, above what's known as the Kármán line. Those satellites you see zipping by in the twilight sky are actually only about 250 miles (400 km) or so away in Low Earth orbit when they're directly overhead, a short day's drive if you could drive your car straight up. The ring of geosynchronous satellites orbiting the Earth once every 24 hours is farther still, at 22,236 miles (35,785 km) above the surface of the Earth. But that's only about 1/10 of the way to the Moon, almost a quarter of a million miles distant.
Moving out into the Solar System, it's handy to simply drop all of those zeros from miles and kilometers and state distances in astronomical units (AU) with 1 AU equal to the Earth–Sun distance of 93,000,000 miles (149,700,000 km). Light, traveling at 186,282 miles per second (299,792 km/s), takes 8 minutes to traverse space from the Sun to the Earth, and just over another second to reflect off of the Moon back onto the Earth.
Distances to the planets take us farther out still. If Mars is out tonight and at closest approach near opposition, it's an average of 49 million miles (79 million km) or 0.53 AU (4.4 light-minutes) distant. Saturn, the outermost naked eye, classical planet, is 930 million miles (1.5 billion km) or 10 AU (83 light-minutes) away.
And Voyager 1 — the most distant object hurled into space by human rockets — was, as of October 10, 2017, 140 AU (13 billion miles [21 billion km] or just under 20 light-hours) distant.
But things scale up again over a thousandfold looking out towards the stars. All of the stars visible tonight are part of the Milky Way Galaxy, which contains perhaps an estimated 400 billion stars. The very closest star system is Alpha Centauri, about 4.4 light-years away. Up north, you can spy Sirius, the brightest star in the sky at about 8.6 light-years away. Most of the stars visible in the night sky, however, are a few hundred light-years away and much larger and brighter than our Sun.
And things scale up another thousandfold, looking outside our galaxy. If you're fortunate enough to live in the southern hemisphere (the South seems to have all of the grand sky objects), then you can spy the cloudy wisps of the Large and Small Magellanic Clouds, irregular satellite galaxies of our own home Milky Way Galaxy, located 158,000 and 199,000 light-years from Earth, respectively.
But don't despair if you live up north. In fact, you can see a bit farther out into the Universe tonight using nothing but the naked eye by following these simple steps:
Look past the upper-left corner of the Square of Pegasus asterism and into the constellation Andromeda. (Fall season in the evening is best.)
Look for the fuzzy patch of the dark sky, 2.5 million light-years away.
Marvel at how that light left that galaxy back in the early Pleistocene epoch, during the heyday of our ancestor Homo habilis.
This represents one of the most distant naked eye objects in the sky.
THE ASTRONOMER'S NEMESES: CLOUD COVER, ATMOSPHERE, AND AIR MASS
It's a deceptively simple term. Seeing — the turbulence or steadiness of the roiling atmosphere of the Earth above your observing site — will, more than anything else, dictate your observing plans, making you decide whether to set up for a night's worth of observing or pack it in for the night. We're fortunate, in a way: Other worlds, such as Venus or Titan, are permanently shrouded in clouds, with no view of the cosmos beyond. What would an observer evolving on such a world make of the Universe beyond these cloud tops?
Will it be clear tonight where you live? Most modern weather forecasts only paint a partial picture when it comes to local cloud cover. Humidity, temperature, seeing, and transparency — a measure of contrast and the ability of fainter objects to punch through the sky glow haze — all play a role.
Amateur astronomers usually assign seeing and transparency a numerical value of 1 to 10, with 1 being a turbulent, murky white sky where stars twinkle and planetary views dance at the eyepiece like you're looking at them from the bottom of a swimming pool, to 10, which represents legendary, rock solid, high contrast, inky black skies with tack-sharp views.
You might see an observer's log reading something like "seeing/transparency = 5/8." These assessments can be a bit subjective, and there are other scales used to assess sky conditions. Keep in mind that you're also looking through a thicker section of atmosphere low to the horizon versus the zenith. This is referred to as air mass, with the clearest, steadiest views being straight overhead near the zenith, versus murky, trembling views low to the horizon. Buildings, parking lots, and bodies of water can also add to the unsteadiness, as they re-radiate the heat of the day back out to the sky and space above. Of course, while we might curse the cloudy skies tonight, we can be thankful our homeworld is insulated with a life-giving atmosphere — it would be a very chilly place, otherwise — but the presence of clouds is the bane of many an astronomer. This is why major world-class observatories are located on windswept mountaintops in Hawaii or in the deserts of Chile, places with unparalleled seeing and clear, transparent skies more than 300 days out of the year.
WILL IT BE CLEAR TONIGHT WHERE YOU LIVE?
Two great sites to watch for cloud cover forecasts tailored for amateur astronomers are Skippy Sky and the Clear Sky Chart.
THE BORTLE SKY SCALE
In addition to the quality of the atmosphere, light pollution is the bane of astronomers everywhere. We can describe how dark or bright the sky is using what's known as the Bortle Sky Scale, an estimate that you can use for your very own observing site (see section in Chapter 11: How Dark Is My Sky? How to Gauge the Quality of the Night Sky,here).
SPLITTING THE SKY
To make sense of the sky, astronomers section it off into groups of stars known as constellations. Constellations are stick figures representing what we see in space and time from our Earthbound vantage point. The constellations in the sky vary from one culture to another: One culture saw a cart and wagon in the Big Dipper, for example, while another saw a plow. The International Astronomical Union currently uses eighty-eight constellations to segment off the vault of the sky in the northern and southern hemispheres. Some of these, such as the twelve zodiacal constellations girdling the ecliptic date back to antiquity, while the southern constellations are newer constructs dating from the age of exploration, starting with the Bayer family of constellations introduced by the German celestial cartographer Johann Bayer in 1603.
Informal groupings of stars are known as asterisms. The Big Dipper, the Summer Triangle, and the Keystone of Hercules are three examples of asterisms. Unlike constellations, there isn't a formal listing of defined asterisms in the night sky; feel free to make up your own.
As with political boundaries, the borders of constellations are human constructs, imaginary lines that help us describe and partition the Universe and the sky overhead. We can say, for example, that a comet is "crossing the border of the constellation Lyra into Hercules" and convey some idea of its position in the sky.
For a more precise position, astronomers use an equatorial coordinate system based on the imaginary positions that the rotational poles of the Earth are pointing at in the sky. This system is like the coordinates of latitude and longitude here on Earth. Again, it's helpful to think of the globe of the Earth embedded in an imaginary sphere of the sky.
Declination is the north–south position of a given object measured in degrees north (+) or south (-) above the celestial equator. The celestial equator is declination 0 degrees, while the poles are at 90 degrees north or south. Standing on the Earth's equator, the celestial equator passes directly overhead through the zenith, while at either pole, the celestial equator traces out the true local horizon.
The right-angle analog to terrestrial longitude is right ascension, usually simply abbreviated as R.A. The celestial equator is divided off into 24 hours of right ascension, each of which are broken down into 60 minutes and 60 seconds. This system arose as a way to make sense of position (mainly at sea) versus a known rising or setting time of an object from a fixed observatory on land, though it really took the advent of reasonably precise chronometers in the eighteenth century to make it work well.(Continues…)
Excerpted from "The Universe Today Ultimate Guide to Viewing The Cosmos"
Copyright © 2018 David A. Dickinson with Fraser Cain.
Excerpted by permission of Page Street Publishing Co..
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
CHAPTER 1 STEPPING OUT TONIGHT,
CHAPTER 2 ASTRONOMY GEAR AND TECH TALK,
CHAPTER 3 FOLLOWING PLANETS AND THE MOON IN THE SKY,
CHAPTER 4 THE DEEP SKY,
CHAPTER 5 THE SKY FROM SEASON TO SEASON,
CHAPTER 6 NEAR-SKY WONDERS: SATELLITES, AURORAS, AND MORE,
CHAPTER 7 COSMIC INTRUDERS: OBSERVING COMETS, ASTEROIDS, AND METEOR SHOWERS,
CHAPTER 8 SOLAR OBSERVING,
CHAPTER 9 ASTROPHOTOGRAPHY 101,
CHAPTER 10 TOP ASTRONOMY EVENTS FOR 2019–2024,
CHAPTER 11 REAL SCIENCE YOU CAN DO AND PROTECTING THE NIGHT SKY,
Appendix I: Constellations of the Sky,
About the Author,