Astronomy excuses for holidays feasting

Featured image: the Christmas Tree Cluster, or Cone Nebula, or NGC 2264. It is a cluster of young stars ionizing hydrogen gas towards the constellation of Monoceros. Credit: ESO.

Astronomers are known for usually not following strict religious traditions, even though some of them still do maintain a certain level of religious belief. Amidst all the frantic activities that the western peoples engage on during their calendar’s last weeks of the year, one might wonder what an astronomer usually do: do they follow the traditions of the holy-days, have their own scientific commemoration or adopt more of a neutral stance? I’m going to give my take on that. But let’s start with a bit of background.

There is an area of study in astronomy that is busy with the position of objects in the sky, and it is known as astrometry (which can be related to astrology – the belief that alignments and positions can either causally or directly affect our personal lives). This is probably the most ancient branch of astronomy, since the days when our ancestors started tracking the position of stars in order to predict the seasons, and consequently help them in the practice of agriculture. Based on the periodicity of the positioning of stars, people created calendars, and there is a plethora of them. The calendar used by the western peoples is Google Calendar the Gregorian Calendar, also known as the Christian Calendar. It is refined a version of the Julian Calendar, which is a refinement of the Roman Calendar, which may have been based on Greek lunar calendars. There is actually a very entertaining history behind all these calendar reformations, so I highly recommend reading about them.

And why does all that matter? Well, you see, the calendar we use today are based on two astronomical phenomena: the seasons and the Moon phases. The cycle of the seasons define the year, while the cycle of the Moon define a month. The week, on the other hand, is probably based on the “enigmatic” number seven, and the days are obviously derived from the cycle of night and day. More recently, we (the human civilizations) noticed that just the positioning of the Sun and the Moon can be a bit erratic in their periods (for a number of reasons), so astronomers defined what is called the sidereal time, which is based on the positioning of background stars in the sky. Now, it’s important to note that even the stars cannot be completely reliable on very long stretches of time (i.e. millions of years), because even they start to move around – our Galaxy is a pretty dynamic system. But in a human lifespan timescale, the background stars are reliable. The point is that the way we measure time and pace is heavily based on the science of positioning objects in the sky, so is there something that makes a day more scientifically “holy” than other?

A few ancient civilizations that kept astronomical records noticed that the Sun had a weird movement on the sky during the year. If one records the exact position of the Sun at the same time, every day, one would get what is known as anallema. This movement affected not only the position of the Sun, but also of the shadows (which are easier to measure and record). And by looking at the position and size of the shadows, our ancestors noticed there were special positions: the solstices (the highest and lowest points of the Sun in the anallema) and equinoxes (the crossing point in the anallema). These phenomena are due to the inclination of the Earth’s rotation axis in relation to the plane of rotation of the Earth around the Sun (but I’m getting a little bit ahead of history here), and the inclination is also the responsible for the seasons. For that reason, many cultures use the equinoxes and solstices to mark the beginning of a season (although the way they actually happen in practice can vary wildly – let’s leave that study for meteorology).

The solstices, or the way we experience them, are very different between the southern and northern hemispheres the farther we go from the equator. Around June 20, we have the northern solstice, or when the Sun is apparently the northern-most in the sky, which means it’s the longest day on the northern hemisphere (and also the official “on paper” beginning of its summer) and the shortest day for the southern hemisphere (also, the beginning of its winter). Around December 20, we have the southern solstice, and everything happens in an inverse fashion. The equinoxes (which happen around March/September 20) are the days when the Sun cross the exact East-West direction, and the duration of the day and night is exactly 12 hours. And here is a fun fact: at the exact time of northern solstice, there are no shadows cast on the tropic of Cancer, while that happens at the tropic of Capricorn for the southern solstice (that’s actually how we define the tropics – I live just one degree north of the tropic of Capricorn); during the equinoxes, that happens if you are on the equator. So this is how the shadows play during these special dates.

The configuration of the Earth and direction of Sun rays during the southern solstice. Credit: user Blueshade on Wikimedia Commons.
The configuration of the Earth and direction of Sun rays during the southern solstice. Credit: user Blueshade on Wikimedia Commons.

Cue renaissance, Nicolaus CopernicusGalileo Galilei and Johannes Kepler. Between centuries XV and XVII, we saw the birth of astronomy as we know today, completely separated from astrology (but not from astrometry – we still do that these days; heck, the mission Gaia recently launched by ESA is doing extremely high-precision astrometry on a 3D space), and the observations of the Solar System led us to construct the theory of heliocentrism, which stated that the planets revolve around the Sun instead of the Earth. Additionally, the observations suggested that the path the planets take are not circular, but rather elliptical, and the Sun stayed (roughly – it actually wobbles just a tiny bit due to the gravity of the planets) at one of the foci (plural of focus) of the ellipse. That means that the distances one planet assumes in relation to the Sun is not constant, they vary; in fact, they have minima and maxima. Which means there are special days during a planet’s year, besides solstices and equinoxes; these are the aphelion (farthest from the Sun) and perihelion (closest). However, the difference in distance between minima and maxima are so small, that they hardly affect the weather here on Earth – which means that they have nothing to do with the seasons – and, in fact, the orbit of the Earth is almost a perfect circle. Almost.

Well then, there you go: in astronomy terms, there are no holy-days, but special days, more specifically six of them: two equinoxes, two solstices, aphelion and perihelion. At the time I’m writing this post, it’s December 21, and the summer solstice (southern hemisphere) happens in just about 8 hours from now. While it marks the beginning of the summer, it’s been very hot since September (temperatures peaking around 30 °C) and, weirdly, it’s been extremely dry when it would normally be the rainy season around here. So I don’t know what summer is anymore, apart from the formal astronomical/calendar definition. Either way, I will be raising a glass of cold water (or whatever I have in my hands) tonight to celebrate this special moment. And this is what I usually do.

Merry southern solstice, and a happy semi-arbitrary turning point of the Earth’s orbit around the Sun for everyone. Enjoy the feasting.

Astronomy excuses for holidays feasting

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