To Look at a Star Is to Look at the Past

Light takes time

Light is fast but not instantaneous. Until that was established, the sky was taken to be the present moment.

Further means longer, so distances in space are counted in light-years — how far light goes in a year. Looking at a star ten light-years off is looking at light that left ten years ago.

A telescope is a bucket, not a magnifier

We are taught that telescopes magnify, yet astronomers hardly mention magnification. They talk about aperture.

Because a star remains a point however much you enlarge it. The problem is brightness, not size. A telescope is closer to a vessel for catching light: a wider surface catches more, and fainter things appear.

A little further in

Seeing with light the eye cannot see

Our eyes accept only a narrow band of light. There is more outside it, and the long-wavelength end of it is radio.

Radio shows different things: what lies beyond dust clouds, and what is too cold to glow. A radio telescope is dish-shaped simply because those waves are long. The job is the same — gathering.

Korea has three radio telescopes, near Seoul, Ulsan and on Jeju. Point all three at the same place at once and combine the signals, and they behave as one telescope as wide as the distance between them.

How distance is measured

Nearby stars are measured by parallax: how far a star appears to shift when the Earth has moved to the other side of its path. Hold up a finger and blink each eye in turn — the same effect.

Further out the shift becomes too small, and brightness takes over: find a star whose true brightness is known, and its apparent dimness gives the distance.

The unexpected work

The institute does not only look at stars.

It calculates and publishes when the lunar new year and the harvest festival fall, and where the leap month goes. The lunar calendar follows the Moon's actual motion and must be worked out each year — a direct line from the days when measuring the sky was how dates were fixed.

The other is watching what might come down: asteroids approaching Earth, and the fragments of dead satellites. Working out where they will fall runs straight back into the orbit article.

Seeing it directly is a disappointment, and that matters

First-time visitors looking at Saturn through the citizens' observatory are usually let down. It is not the spectacle from the pictures — a small, faint yellow dot with a ring.

But the disappointment is the point. Photographs are long exposures, many frames combined, colour applied. That small dim dot in the eyepiece is real light that left Saturn about an hour ago and entered your eye.

That is the difference between a screen and an eyepiece. Faint as it is, this is not a broadcast.

What you would study to do this

This is not a neighbourhood of astronomers only.

1. Someone who decides what to observe and what the result means — astronomy.
2. Someone who rebuilds the universe inside a computer — computational astrophysics.
3. Someone who grinds mirrors and builds and drives telescopes — optics and mechanical engineering.
4. Someone who builds instruments that catch faint signals — electronics and detectors.
5. Someone who sifts the flood of observations — computing and statistics.
6. Someone who runs the observing night after night and keeps the equipment alive — observatory operations.
7. Someone who tells visitors what they are looking at — public astronomy interpretation.

Do not pass over that last line. No telescope, however fine, tells you what the point of light is or when it set out. Someone has to say it, and only then does a star become a star. What people remember longest from an observatory is usually not the lens but whoever explained it that night.

The work is done at the Korea Astronomy and Space Science Institute in Yuseong-gu, Daejeon, and the eyepieces are at the Daejeon Citizens' Observatory.

The question that remainsIf everything we see is slightly in the past — where exactly is the moment we call now?