What is that spot in the sky? And what is it doing?
How visual astronomy has advanced! Astronomers find one tiny dot in millions of others, and figure out what it is.
When I was a boy I learnt a great deal about the stars from my father. He was an amateur astronomer, but also a herpetologist with a snake laboratory in the Indian jungles. There the nights could be breathtakingly clear and the stars would shine like bright jewels in the sky.
One of the things we did in those dark nights was to test our eyesight — and that of friends and visitors — on the Mizar-Alcor binary system. It is located in Ursa Major, the Big Dipper, a well defined constellation that is easily spotted. People with very good eyesight can see that the second-last star in the handle of the dipper is actually two stars. We would test whether they could really see this by asking them “what o’clock?” They had to tell us in what position on a clock the smaller star, Alcor, was located in relation to the brighter binary Mizar.
My eyesight was pretty good, and I could easily spot Alcor. You should try doing it yourself on a clear night. I discovered by looking slightly away from stars I could better identify binary systems. It is a technique called averted vision and works, my father explained, because the rod cells in the retina, which are more sensitive to faint light, are more concentrated away from the center of your vision. Averted vision also allowed me to spot some globular clusters and nebulae, even the constellation Andromeda.
As my interest in astronomy grew, and seeing my disappointment in not being able to see binaries and multiple star systems, my father decided to build me telescope. I got a refractor with a three-inch primary lens.
The telescope gave me a spectacular view of the Moon, and I could see the rings of Saturn, as well as the four Galilean moons of Jupiter, which I followed night after night. The stars however were somewhat disappointing: they were always just points of light — never disks. But at least I could locate and view dozens of well-known binaries, which I duly did.
An important thing I learnt from my use of the telescope was how many stars there are. In well-lit cities you could, with the naked eye, see just a few dozen, in suburban areas a few hundred, and on a clear night in the jungle, especially after rainfall, around 5000 stars were visible. But through my telescope I could see many tens of thousands! The entire sky was carpeted with stars, there were no empty spaces.
What does the sky look like with bigger and better telescopes, with ten meter mirrors? You see millions of stars and still no blank spaces. The galaxy is full of them. Take a look at what a tiny section of the sky looks like through a really big telescope:
Yes, all the tiny dots in the image, are stars in our galaxy. The small red square of stars is enlarged, and that is where astronomers found a tiny little dot that left them spellbound.
What could be so special about this one dot amongst a million others? And how did they find it? Well, the dot moved! In multiple exposures astronomers found that it was changing its position.
Regular stars in our galaxy are so far away that they stay fixed in their positions in the sky. So this was clearly something within our solar system, an object that was moving and reflecting light from the sun. A comet, an asteroid?
Astronomers were able to determine the path of the object — thanks to our knowledge of the laws of gravity, worked out by a 23-year-old lad about 350 years ago, and by improvement in the mathematics of these laws, devised by a 24-year-old 140 years later. I refer to Isaac Newton and Carl Friedrich Gauss, and the adventure of celestial calculation described here.
Using these tools the astronomers quickly realized that they were dealing with an interstellar visitor, only the third we have spotted in our solar system. They dubbed it 3I/ATLAS and found it was moving in an unbound, hyperbolic trajectory around the Sun.
The diameter of the object is between one and 24 kilometers, and it is moving at a speed of 61 km/second. It’s closest approach to earth will be 170 million miles, so we don’t need to worry about a Chicxulub-type extinction event. It is just an opportunity to study a galactic visitor, one that might quite likely be twice as old as our solar system.
And this is how far we have come in 500 years — after Galileo trained his three-inch telescope at the Milky Way and discovered it was made up of stars!
Addendum December 19, 2025
Today 31/ATLAS makes its closest approach to Earth — it will wiz by at a mere 269 million kilometers.
Astronomers originally estimated that the icy core was tens of kilometers across, but Hubble’ has narrowed it down to no more 5.6 kilometers. Astronomers think it could even be just over 300 meters.
