Ask Ethan No. 33: Flight Without Stars

Do not cling to particulars, but strive to fly up to where an open view of the whole problem opens up, even if this view is not the clearest.
- Ludwig Wittgenstein

The reader asks:

On a clear night in the city of Yonkers, I see several stars. In Monroe, I see even more. Why can't I see them while on a plane above the clouds? I thought there would be a planetarium right ??

A difficult problem. Let's think about it.



From places with strong light pollution (most cities), only the brightest stars and planets are visible. Starlight has to compete with all other light sources. In New York, Chicago or Los Angeles, the sky is so heavily lit that you can only see a couple of dozen stars.

This pollution is so strong that you will not even notice the difference in the presence or absence of the moon in the sky.



For areas farther from the city, this is no longer the case. Of course, there is some light pollution, but the number of visible stars will increase from a few tens to a thousand. In addition, the brightest nebulae, such as the Andromeda nebula, can be seen with the naked eye in the absence of the moon. The difference is huge - but not so much when compared with the case when you find a really dark place to observe.



The number of visible stars increases to numbers from 6000 to 45000 , the Milky Way becomes visible, and it becomes possible to distinguish the colors of stars.

But there are also " super dark " places.



We measure the darkness of the sky on the Bortl scale. The color of the Milky Way and such rare species as the zodiacal light and anti-radiance (sunlight reflected from dust located in the plane of the solar system) are noticeable in a super-dark sky.

But even with such a sky, a simple natural phenomenon can prevent you.



These, of course, are clouds. And also strong turbulence in the atmosphere.

All of these factors - light pollution, clouds and turbulence, affect what an observer can see. And when using a telescope, these factors will affect the observations even more. Therefore, for good observations it is necessary to go to such places.



You find a high place above the line of clouds and most of the atmosphere. It is far from cities and other sources of light pollution, and where the air behaves calmly.

Better this will only launch the telescope into space.



Mindful of the above, one would think that a night flight by plane at an altitude of 9 kilometers above the ground is the perfect time to observe the sky.

But if you looked out of the plane at night, you could see the moon, a couple of planets - and that’s it. Not counting the lights on Earth.



But why?

It is quite easy to understand if you recall the night spent in your house. If you turn on the light inside and the outside is dark - what and where will it be seen?



The observer outside the house sees everything that happens inside. But to see what is happening outside, you need to turn off the light in the house.

Usually, the place you are looking from should be darker than where you are looking.

So what's up with the plane?



There is always light on the plane and this greatly limits your ability to observe. And this is the only reason that you do not see the night sky from an airplane.

If you persuade the team to turn off the light in the cockpit, or you got yourself a release aircraft before 1940, without backlighting, you can see the same sky as in the next photo.



In the early years of aviation, pilots took sextants with them and navigated by the stars - just like sailors in those days when there was no GPS. Here's a picture of a navigator reading a sextant from a 1940 British military aircraft.



If it weren’t for the light on the plane, you would have watched the sky at its best. Astronomers know this, and NASA even has a special telescope mounted on an airplane - SOFIA .



So, if you want to see the stars from the airplane window - ask the team to turn off the lights in the cabin, and the Universe will be yours!