More Twinkling Stars
Atmospheric scintillation, the scientific term for the twinkling of a star, is, in truth, a form of optical illusion, the consequence of the interaction of its light with the Earth’s atmosphere. The Earth’s atmosphere, starting about 10,000 kilometres above its surface, is made of different layers and is affected by winds, varying temperatures, and different densities. Travelling from one medium to another, in this case from outer space through the Earth’s atmosphere, the light from a star bends many times, a phenomenon known as refraction. By the time we see it, the refractions have caused the rays to zigzag, some reaching us directly while others have bent either away from or towards us. So quickly does this happen that the star seems to twinkle.
The degree to which a star will twinkle will depend upon its place in our field of view and weather conditions. Stars near the horizon will appear to twinkle more than those directly overhead because the light has to travel through more of the atmosphere to reach us. Humid nights will also cause the air to be thicker, making stars appear to twinkle more.
Ironically, the conditions which induce the stars to appear to twinkle more and delight the casual observer are an anathema to the astronomer. Thick air which causes more atmospheric scintillation results in what they call “bad seeing”, while conditions where the air is thin, dry, and calm and interference is minimised result in “good seeing”. Astronomers tend to site their observatories in high, dry areas to remove as much as air as possible between the stars and the telescope to minimise interference.
Unlike a star, a planet does not twinkle. While the distances in space seem astronomical to the layperson, planets are significantly closer to the Earth than stars. The furthest planet in the Solar System, Neptune, orbits the Sun at an average distance of 2.794 billion miles and is approximately 4.2 light hours or 0.000475 light years away from Earth. In contrast, the nearest star to Earth, Proxima Centauri, is over four light years away.
This difference in relative distances affects the way we perceive light. A star appears as a single point of light which dims and brightens as it refracts on its way through the atmosphere. The light from a planet, though, appears to come from more than one point source and the dimming of some of its rays caused by refraction is counteracted by the brightening of others, giving the appearance of a steady, unblinking source of light.
Worryingly, Shakespeare’s night sky “painted with unnumber’d sparks” (Julius Caesar, Act 3; Scene 1) might soon become a thing of the past, a consequence of the light pollution, a phenomenon first recognised in the 1970s, caused by the growing use of ever more powerful artificial outdoor lighting. Based on 50,000 observations made between 2011 and 2022, a recent report in Science reported that the night sky is getting lighter by an average of 9.6% globally each year. If that rate continues, over an eighteen-year period the night sky would get brighter by a factor of four or, to put it more graphically, a child born today where 250 stars might be visible would only see a hundred of the most brilliant by the time they were eighteen.
The rate of increase is not constant throughout the world, though. In Europe, it is increasing at a rate of 6.5% per annum, below the global average, whereas in North America the rate is 10.5% and an estimated 8% in Africa and Asia. Even so, according to the Light Pollution Atlas, the Milky Way is no longer visible to 77% of Britons.
The problem is now being recognised with National Parks and Councils appointing Dark Sky officers to maintain and improve the quality of the night sky in their areas. The recently published Ten Dark Sky Policies for the Government, calls for stricter controls on the use of outdoor lighting installations and for greater emphasis to be placed on recognising the impact of light pollution through education, incentivisation, and regulation.
Whether these proposals will see the light of day only time will tell. In the meantime, savour the twinkling stars while you can and switch off any unnecessary outside lights.
