We wouldn’t be able to see much when we looked up if the night sky wasn’t dark, and the specifics of how many and what kind of celestial objects we can see depends on how dark the sky is.
Its darkness is quantified by the Bortle Scale, which grades the darkness of your sky into one of nine levels. The darkest skies are Level 1 and feature a diverse group of objects from naked-eye Messier clusters and the Milky Way to the sunlight delicately scattered by interplanetary dust.
As the sky gets less dark due to light pollution, its grade decreases on the Bortle Scale and along with it, the number and kind of celestial objects reduce as well.
While there are several astronomical views we can enjoy in the city, others require a dark sky. Use our guide (which opens in a new tab) to find the darkest sky location in your state.
In this article, we will learn of five astronomical sights for which dark skies are a must.
Zodiacal Light
Definition and Cause
You might know that the leftover gas and dust from the solar system’s formation is suspended in the same disk as our system’s planets.
When sunlight hits these dust particles and the particles reflect its rays, we see the phenomenon from Earth as a hazy, slightly white pyramid of light called the zodiacal light. The name is inspired by the fact that this dust is located in the zodiac.
Visibility
The best times to see zodiacal light are after twilight and before dawn. This phenomenon is visible in the west after sunset and in the east before sunrise.
For those in the northern hemisphere, the best views are around the spring and fall equinoxes. We’ll see it in the morning from late February to early April as a ‘false dusk’, and from late August to early October as a ‘false dawn’.
Gegenschein
Definition and Cause
Gegenschein is similar to zodiacal light in that it is also caused by the scattering of sunlight by dust particles in the ecliptic.
However, unlike the zodiacal light that’s a hazy pyramid, the Gegenschein is a fine point of light directly opposite to the Sun.
It is also distinguished by its brightness, which occurs because, at that angle, the maximum amount of sunlight is reflected in the same direction through a concept called backscatter.
Visibility
Viewing the Gegenschein is quite a challenge with the naked eye because it requires the observer to guess the direction opposite to where the Sun would be.
The easiest of all timings is looking directly overhead at midnight, knowing that that is 180 degrees from the Sun.
The Milky Way
Definition and Cause
The Milky Way is the beautiful galaxy where our home – the solar system – resides. Its diameter of more than 100,000 light-years encompasses over 100 billion stars.
Like all galaxies, scientists think the Milky Way was created about 14 billion years ago from collapsing gas and dust in a similar process that created our solar system, albeit on a much larger scale.
Visibility
Since we are inside the Milky Way, we can only see part of it from our viewpoint on Earth.
For half a year, the Milky Way’s core is not visible from the northern hemisphere because it is below the horizon. The times when we can see it are given below.
- December through February: not visible
- March through May: before sunrise
- June through August: all night
- September through November: before sunset
This is an easier object to see than the ones we’ve looked at so far. Find it stretching from the northeast to southwest horizons, passing directly overhead in the middle of a summer’s night.
It passes through the constellations of Auriga, Perseus, Cassiopeia, Cepheus, Lacerta, Cygnus, Aquila, Scutum, and Sagittarius, as the star chart below, from SkySafari 6, shows.
Airglow
Airglow is light that occurs when the particles in Earth’s atmosphere that are ionized by sunlight throughout the day recombine at night. Similar light is also emitted by outer space cosmic rays striking our atmosphere.
While the sunlight’s glare masks this phenomenon during the day, it comes to life at night as a bluish light in the sky — similar to aurorae but much lower to the ground. The other colors are red and blue.
Airglow appears 50 to 300 km or 31 to 180 miles above the ground.
Aurorae
Definition and cause
While also a phenomenon of light, aurorae are different from airglow in how they are created and the height at which they appear.
Aurorae, commonly known as northern and southern lights, occur when the Sun’s charged particles (or solar wind) enter the Earth’s atmosphere through its poles, where the planet’s magnetic field is the weakest.
Once inside, they react with the atmospheric elements and produce beautiful colors.
Visibility
They are best seen closer to the Arctic Circle (Canada, Sweden, Norway, etc) on the north and Antarctic Circle in the south, although the north is more popular for astrotourism.
Aurorae in the north are known as Aurora Borealis, those in the south as the Aurora Australis.
When to see them depends on the solar cycle. While aurorae occur every night, they are more bright and intense during solar maximum, the time of most sunspot activity, because that’s when the Sun spews more charged particles, which in turn result in more intense aurorae.
Summary
The beautiful phenomena outlined above can only be seen when the skies are at their darkest.
As light pollution continues to reduce the number of stars and other celestial objects that we can see, delicate dark-sky-only phenomena such as airglow, aurorae, and zodiacal light among others, remind us of the importance of treasuring the pristine skies.
Which one of these beautiful astronomical sights would you watch first?
