Let there be twilight: brighter nights in Belfast and beyond
Hiya! Ceren here from Datawrapper’s customer support and success team. This week, I’m taking a closer look at a luminous topic: the different types of twilight.
A couple of weeks ago, some friends and I gathered in the woods to celebrate Bealtaine, one of the four fire festivals between the solstices and the equinoxes that mark the four seasons, according to the Celtic wheel. Bealtaine marks the beginning of the lighter half of the year. Aptly, we lit our fires, barbecued, sang songs, and felt relieved to be finally leaving behind a horrid, incredibly wet winter, including a record-breaking 40+ consecutive days of rain in parts of Ireland.
The night carried on, but it never truly got dark. Coming from a much more southern latitude, where the sun sets no later than 8 p.m. even at the height of summer, I am forever fascinated by how bright Belfast remains in the evenings, and by how long that light lasts.
The three stages of twilight
That night, my friend Ellis revealed to me that night and day are not as clear-cut as black and white. The day is as we know it: whenever the sun is above the horizon. But before the day turns into “true night”, where the sky is entirely dark, and the naked eye can detect the faintest of the celestial objects, wedged between the two are several stages of twilight, each defined by how far the sun sits below the horizon.
Twilight is the transient period of time where the sun is below the horizon, but there is still some sunlight being scattered and refracted in the sky. More specifically:
Civil twilight refers to the brightest form of twilight where the sun sits less than 6 degrees below the horizon. The sky is typically bright yellow and orange, and artificial lighting is not necessary for most outdoor activities. Because of that, the cut-off point for this particular type of twilight is sometimes used to make laws related to aviation, hunting, and the usage of headlights and street lamps.
Nautical twilight dates back to sailors using the stars to navigate the seas, as most stars can be easily seen with the naked eye.
Astronomical twilight is dark enough for most celestial objects to be observed, but a small amount of sunlight is still refracted in the sky.
Bright summer nights in Belfast
On nights like the one when we celebrated Bealtaine, Belfast never reaches true night, meaning the sun never dips more than 18 degrees below the horizon. The darkest it gets is astronomical twilight, when there is still some sunlight scattered across the sky; not enough to orient oneself in the outdoors, but enough to unsettle those of us who aren’t used to the phenomenon.
On the nights to come, that brightness only intensifies. In the Northern Hemisphere, the days keep getting longer until the summer solstice on June 21st. And here in Belfast, there’s a short spell around that time, June 19th to 25th, when not only do we not get any true night, we don’t even get astronomical twilight.
True night around the world
For many people in Belfast, this is just how the days and nights have always changed. But I find it fascinating because it’s not something I grew up with. So, while reading more about the different kinds of twilight, a question naturally came up: where else in the world are there days without true night, and for how long does that persist?
In the Northern Hemisphere, this phenomenon first appears at around the latitude 49°N¹. As we move farther north, the number of days without true night per year increases.
Conversely, as we move farther south, true night persists throughout the year. Paris, for instance, at roughly 48.85°N, never experiences a day without true night. (Neither does my hometown of Adıyaman, Türkiye, at 37.76°N.) This is true until we reach about the latitude 49°S, where days without true nights become commonplace again, and their number increases as we continue farther south.
If you hover over the map above, you might notice that just above the cut-off, the number suddenly jumps from 0 days to over 30, with no gradual build-up. I’ve spent a fair bit of time trying to understand this and have gathered that it’s due to a combination of solar declination (the angle between the Sun’s rays and the Earth’s equator) changing very slowly around the solstices, and the Earth’s axial tilt. That said, it feels like I’m only scratching the surface.
So instead, I’m going to leave it there and enjoy this Belfast day with a whopping sixteen and a half hours of daylight and zero hours of true night!
That’s all from me — thanks for reading! Come back next week, when the days are even longer (if you’re in the Northern Hemisphere), for a Weekly Chart from my colleague Moritz. Until then, I hope you enjoy the sunshine!
Notes
¹ This particular cut-off point and the values depicted in the map reflect a 1° resolution grid with centres at 0.5° intervals (e.g., 49.5°, 50.5°). As such, the values may differ slightly from other calculations. So, if you live near this threshold, it might be worth double-checking using other resources.
