What was that? The color-changing, flickering “UFO”: Sirius

6 min read

It sits low in the southeastern sky, flickering red, green and blue, and on a phone video it turns into a pulsating disk. Many people in this situation are sure they are seeing something unusual. Most of the time it is Sirius, the brightest star in the night sky. We show why it is so striking, why the camera is fooled too, and how you can check it in a few minutes.

What does it look like?

A single, very bright point. Sirius has a magnitude of –1.46. It is the brightest star in the night sky, almost twice as bright as the runner-up. Only the Moon and a few planets (Venus, Jupiter, sometimes Mars) outshine it.
It stays low. From Hungary it never rises higher than 26 degrees, so it always appears near the horizon. At its highest it is roughly two and a half fists above the horizon, measured at arm's length.
It flickers and changes color. Even with the naked eye you can see that it vibrates rapidly and flashes red, green and blue for moments. Through a telescope or on camera this is even stronger.
It moves slowly and steadily. It rises in the east-southeast, reaches its highest point in the south, and sets toward the west-southwest. Its motion is so slow that it seems to stand still for minutes.

Why does it fool people?

The atmosphere's “lenses.” Starlight passes through air pockets in the atmosphere that differ in temperature and density, and each of them deflects it slightly. The air pockets move, so the star's image jumps around and changes brightness several times a second: this is scintillation. Planets twinkle less because their apparent disks are much larger, which averages out the shimmer.
Colored flashes. Air refracts light of different colors by slightly different amounts. So a passing eddy of air steers red light into your eye one moment, green or blue the next. This is most striking with white stars, and Sirius is white.
Much more air near the horizon. Near the horizon, light travels a path through the atmosphere up to ten times longer than overhead. Because Sirius is always low from Hungary, it almost always flickers strongly here. Where it rises overhead, it is much calmer.
The camera turns it into a “disk.” In the dark, the autofocus of a phone or camera cannot lock onto a tiny point of light. It hunts back and forth, so in the footage the star occasionally swells up, becomes a pulsating, colored disk or “dome,” and then turns back into a point. Digital zoom makes this worse.
It seems to move. If you stare at a lone point of light in the dark for a long time, tiny movements of your eyes can make it seem to wander back and forth. This is the autokinetic effect. If you film the star handheld, your shaking hand “moves” it even more.

Real cases

Tynemouth, England, January 2015. A man walking his dog filmed a light that he described as an unusual color and that, according to him, did not move like an airplane. Zooming in, he saw it as a dome with a bulge in the middle and pulsating lights. According to an astrophysics student at the University of Sheffield, the most likely explanation was Sirius: atmospheric distortion makes it appear to flicker and change color, and the student said it was just the eye being fooled.
Maryville, Tennessee, USA, January 10, 2019. A woman sent two videos of a light at dawn to the tip line of the UFO archive The Black Vault. The amateur astronomer who analyzed the footage used the Stellarium planetarium program to reconstruct the time and direction: the “object” was Sirius. In the video the star went in and out of focus because the camera's autofocus was hunting. The case file has since been listed as: solved, star.
A planetarium, 2014. American astronomy writer Bob King described how a man brought a video to a local planetarium of a UFO that, in the footage, turned from a star into a swollen disk and back again. Based on the time and direction, the staff identified Sirius, and the effect was caused by the camera's focus hunting. The man could not be convinced.

How can you check?

Check the direction and the time. If the point of light appears low between the southeast and the southwest, no more than about 26 degrees above the horizon, and the season also fits (see below), it is very likely Sirius.
Find Orion. Sirius lies down and to the left along the line extended from the three stars of the “belt” of the constellation Orion. If it is there, you have found it.
Use a star map app. Stellarium, SkySafari or Star Walk 2 will show you which star you are looking at when you hold your phone up to the sky.
Wait ten minutes. If the point is in the same direction, just a little farther west, it is a star. An airplane or a drone would have long since disappeared in that time.

What to expect in the coming months

Right now, in the fall, Sirius is still in the predawn sky, and it rises earlier week by week. In winter it is visible in the evenings at a convenient time, so from December to March many reports about it are expected. According to our own calculations, this is how it looks from Budapest (rising times accurate to within about 10 minutes, depending on the height of the horizon):

Mid-October: rises after midnight, culminates around 5:30 a.m., about 26 degrees high in the south.
Mid-November: rises around 10 p.m., culminates at 3 a.m.
Mid-December: rises around 8 p.m., culminates at midnight.
Mid-January: rises around 6 p.m., culminates before 11 p.m. This is the best period: after dinner it shines in the southeastern sky.
Mid-February: it is already up at dusk and culminates before 9 p.m.

It flickers most strongly just after rising or when it is about to set and is very low. It is most striking on humid, windy, cold nights.

Quick identifiers

A very bright, white point low in the sky, toward the south.
Flickers rapidly, red, green, blue.
Does not noticeably move even over minutes.
Lies down and to the left along the line of Orion's belt.
On camera a “pulsating disk,” to the naked eye a point.

The sober take

A color-changing, pulsating light near the horizon is in most cases a bright star, most often Sirius. The flickering and the colored flashes are caused by moving air pockets of differing density in the atmosphere, and the “disk” seen in footage is caused by the camera's focus hunting. From Hungary, Sirius is always low, so here it twinkles especially strikingly. If a point of light stays in the same place for minutes and lies along the line of Orion's belt, it is almost certainly not a UFO but a star 8.6 light-years away.