Why do astronomers look at the sky upside down and reversed?

Question

When you look through binoculars, you look clear directly, but astronomers looked other way contrarily. Why is it so?

Answer 1

All these so-called "Top Contributors" in Astronomy & Space who have answered this question so far are :

(a) WRONG about what astronomical telescopes actually do. Your description is in fact better, though it can be improved;

and

(b) NOT ONE of them has actually ANSWERED your question, "Why do astronomers look at the sky upside down and reversed?" --- they have merely repeated (though in an incorrect way!) what you have asserted. Let's deal properly with both of these issues:


(a) FIRST, let's get rid of that old canard that people bring up, propagating error unthinkingly. Astronomical telescopes DO NOT "INVERT" IMAGES, THEY ROTATE THEM !

Just think about it for a minute. Is there ANY way that a rotationally symmetric lens would know in which way gravity was pointing, or where "up" and "down" were ? (You should answer that rhetorical question with a resounding "OBVIOUSLY NOT !")

They do, however, know where the axis of the lens system is. What happens is that EVERY POINT in the original source appears in the image on the OPPOSITE side of that central axis. That means that EVERY angle theta measured with respect to some fiducial line of theta = 0 has 180 degrees added to its angular measure. But
theta ---> theta + 180 degrees is also simply a ROTATION through 180 degrees.

So, BINGO : in their overall effects, astronomical telescopes ROTATE images by 180 degrees !

I hope that the scales are now dropping from some responders' eyes.

What this ROTATION of the image means is that top and bottom will be interchanged, AND left and right will be interchanged (which is your own description of "look[ing] at the sky upside down and reversed"), but then so will ANY two parts of the original scene that were on opposite sides of the telescope's axis. As I've indicated above, the net effect of all these "through-the-axis-reversals" is PRECISELY a ROTATION of the image, which is a much better way to think of it.

The key point to remember is that the image is not JUST "inverted" (which suggests simply an up-down transformation), it is rotated.

That's dealt with point (a). I now deal with the remaining point:


(b) Why are astronomers content to do things in this strange, rotated image way?

The answer is that the light coming in from the Universe is extremely feeble, and astronomers want to minimize the losses due to every unnecessary additional reflection or refraction.

The first images formed by either a simple two-lens or reflecting mirror telescope are indeed ROTATED, but they are brighter and have less chance for further losses or distortions that might occur with additional lenses or mirrors to ROTATE the images to the "right orientation" again. That is only needed for a human's convenience, but it is simply not necessary when using a photographic plate or a CCD chip (which stores the information from incoming photons electronically).

After all, once the astronomer has TAKEN the photograph, it can be ROTATED through 180 degrees and it will then look EXACTLY like the original scene, or the spectrum that he or she may have been interested in will be what we now consider to be "the right way up" ! ###

I hope that everyone who has read this full answer will now feel thoroughly chastened. You can redeem yourselves by going out into the world and preaching the gospel according to Dr Spock:

ASTRONOMICAL IMAGES ARE ROTATED, NOT "INVERTED!"

Live long and prosper.

### HISTORICAL NOTE. Astronomical spectra are all displayed nowadays with the shortest ("bluer") wavelengths on the left, and the longest ("red") wavelengths on the right, which corresponds to having the wavelength ("lambda") increasing to the right like a typical x-variable.

However, Fraunhofer (1787 - 1826), the brilliant young optician and discoverer of the many lines now named after him in the solar spectrum, lived before the age of photography. He made beautiful watercolour paintings of the solar spectrum. But they were all done and labelled in the OPPOSITE direction from today's spectra. Why? Well, it was simply an artifact of how his spectroscope delivered the spectrum to his view. He had no particular reason at that time for painting it in the opposite (reversed) way that would satisfy today's sensibilities.

What is more, Fraunhofer was actually RIGHT, in a very fundamental way. Although people talk of light of a certain wavelength, that is paying attention to a SECONDARY property of light waves. The WAVELENGTH of light depends upon what MEDIUM it is travelling through!

The FUNDAMENTAL property of light is its FREQUENCY. The frequency of light is determined at the moment of emission, and like having a cheerleader marching with a band, the waving baton of the waves (their time-derivative properties) drives the waves continually forward with the same temporal behaviour. The only ways that this will change are the usual ways that happen for any time-based phenomema, i.e. with an observer in relative motion, or moving into regions of spacetime where local time itself runs at different rates.

So: Fraunhofer's original, classic paintings of the solar spectrum were actually oriented, labelled and therefore aligned in the direction corresponding to that FUNDAMENTAL property, the FREQUENCY of the light, with that frequency increasing like an x-variable.

About a century later, the fundamental significance of frequency was underlined by Planck discovering his own fundamental relationship :

E = h nu, where ' nu ' is the frequency.

Answer 2

it particularly is slightly perplexing to assert the image will advance its left hand. permit's say it will advance its hand this is on the superb suited area. Then each and every thing is wonderful because of the fact the image's head is upside and the raised hand on the superb suited. the only axis this is reversed is the only perpendicular to the mirror's floor. whether it particularly is a great factor. the rationalization why this grew to become into perplexing is that our bodies are just about symmetric to this turn, so as that the image feels like a human grew to become in the direction of us. If we weren't so symmetric, we does not think of so.

Answer 3

It's because that is the way the lenses or mirror reflects the light entering the telescope. In order to right it so it is not upside down and reversed an extra lens or mirror would be required. Each lens or mirror lessens the amount of light to the eye or sensors so to minimize the amount of light lost they leave the image upside down and reversed.

Answer 4

The star maps are made in such a way that they appear as you lye-down on your back keeping your head towards North and look-up. That is why you will see East on the left side of North in star maps unlike normal maps where you find East on the right side of North.
This is comfortable to study stars. Just lye-down on your back and look up into the map and sky alternately.
If you are talking about what you see through telescopes, yes in Newtonian telescopes you will see objects upside down.

Answer 5

They don't, but the optical systems of most telescopes consist of 2 or more lenses and they can invert the images captured. Reflecting telescopes also invert the images due to the way the optics are built and the physics of lightwaves.

Answer 6

Telescope lenses and mirrors reverse the images, and we just got used to looking at it that way.

Answer 7

I think that they use lenses and mirrors that do flip the image, but by the time it reaches their eye it has been reflipped to the correct orientation.