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How the Kodaikanal Observatory Tracked Sunspot Activity Over 115 Years

How the Kodaikanal Observatory Tracked Sunspot Activity Over 115 Years

On January 9, a group of astronomers from around India uploaded a preprint paper analysing images of solar activity, encompassing 11 solar cycles, obtained from the Kodaikanal Observatory.

If this sounds unremarkable, you’re probably not aware that each solar cycle lasts eleven years. So 11 solar cycles stands for over a century.

The Kodaikanal Observatory in Tamil Nadu, India, has carefully tracked sunspot activity over the last 115 years, using a combination of hand-drawn images, photographic plates and, more recently, films.

Its archives in effect provide a tantalising glimpse of the history of astronomical methods, each epoch defined by the most common way stargazers recorded their observations of the sky.

Axiomatically, these records also emphasise the extent to which computers have revolutionised modern astronomy. When Galileo Galilei discovered some of the moons of Jupiter and spotted Saturn’s rings, he had pencil and paper — not photographic plates and imaging software. So while he observed some planets, moons and stars, and found the Milky Way to be a densely packed stellar cluster, the CHIME telescope simultaneously observes 1,024 points in the sky at 16,000 frequencies sampled at 1,000 Hz, connected to a computer that can process data at 13 TB/s.

However, such computing power emerged only in the mid-20th century, if not later, whereas our Sun changes on a timescale that effortlessly transcends the epochs of human technology. So the Kodaikanal Observatory’s images of sunspots represent an important historical resource that allows us to understand our star’s (minuscule) evolution over the course of a century.

Sunspots – which appear as dots on the surface of the Sun – are darker than their surroundings because they are cooler, a tepid 4,500 K instead of 6,000 K. The astronomers who first observed these features meticulously recorded their details in drawings. For example, Thomas Harriot, a contemporary of Galileo, drew the following in December 1610:

Observing the Sun without protective eye-gear is extremely dangerous; observing the Sun through a telescope without an appropriate filter is even worse. However, one easy way to get around this problem is by projecting images of the Sun onto screens or walls, irrespective of the star’s brightness.

When scientists cast these projections on photographic plates, they could finally photograph the Sun, in effect preserving a near-permanent record of the solar surface without having to second-guess their way around the idiosyncrasies of human illustrations.

Far from being the static, steady source of light and heat that Aristotle presumed, the Sun is a dynamic, even volatile object. Protons slam into each other inside its core; superhot ions are tossed in its convective outer layers; and solar flares erupt from the surface.

The movement of the trillions of charged particles on the Sun’s surface creates little magnetic fields that line up across great distances. In some areas, where the field lines are particularly close together, they trap electromagnetic energy between themselves and keep it from being radiated into space. These regions appear darker.

Sometimes, the field lines twist and become entangled with each other. The rules of physics disallow field lines from intersecting each other, so while disentangling themselves, the fields abruptly snap into more stable positions and release a large amount of radiation into space as solar flares.

A solar flare visible (towards the left edge) in this X-ray image of the Sun on February 24, 2014. Photo: NASA/SDO

A sufficiently strong flare can disrupt radio communication in Earth’s atmosphere. If the radiation is accompanied by high-energy charged particles, the phenomenon is called a coronal mass ejection. These charged particles interact with Earth’s upper atmosphere to trigger beautiful auroras in the skies over the planet’s northernmost and southernmost countries — and mess with electricity transmission grids on the ground.

Solar activity is periodic. The Sun’s magnetic field reverses its polarity – i.e. flips its orientation – once every 11 years. In this time, new sunspots appear on the surface and others disappear; some even move around. At the beginning of each solar cycle, in a window called the solar maximum, sunspots occur more frequently, at higher latitudes and in greater numbers, indicating high surface activity. Eventually, their population diminishes towards the solar minimum, heralding the decline of surface activity.

When the number of sunspots in each hemisphere is plotted against time, a butterfly diagram takes shape. Each diagram depicts solar activity in each successive solar cycle. Image: NASA
An example of a butterfly diagram composed using Kodaikanal Observatory data. Photo: Indian Institute of Astrophysics

At the Kodaikanal Observatory, to register the relative position and age of each sunspot, an astronomer takes the film with that day’s images and projects them onto a Stonyhurst grid. When a sphere is placed in this grid with its north direction pointing vertically upwards, the grid represents latitudes as horizontal curves and longitudes as vertical curves. Each line in the Stonyhurst grid marks 5º of latitude or longitude.

A Stonyhurst grid with gridlines every 15º. Image: Stanford University

Since the Sun rotates about its axis once every 27-28 days, an astronomer can calculate the slight difference between the rotation periods of its northern and southern hemispheres by measuring the time taken for sunspots in each hemisphere to travel across the disk. Next, the astronomer marks the central longitude number of that day for reference.

Also read: How Earth’s Magnetic Shield Was Breached – and a Telescope in Ooty Tuned in

Four images are recorded on the same grid and colour-coded for reference. The borders are traced in blue ball pen, solar filaments are sketched in red pencil, limb prominences are drawn in blue pencil and sunspots, in HB marker pencil.

To ensure the data is consistent and uniform, the same observer is expected to perform this exacting task every day, at least as often as is possible. Each image thus produced is carefully stored in a bound booklet called a Sun chart. Each Sun chart contains six months’ worth of observations. Astronomers analyse them to understand the ebb and flow of each solar cycle.

Sunspots visible on the Sun’s surface in this 2012 photograph. Photo: bernd_thaller/Flickr, CC BY 2.0

This way, the Kodaikanal Observatory has been closely following the Sun’s activity as well as creating an important historical resource that memorialises both astronomical observations and how humans endeavoured to obtain and record information of the cosmos in a time before computers changed astronomy. The observatory recently digitised its images as well.

When the authors of the January 9 preprint paper calculated the number of sunspots from this archive, between 1905 and 2016, and compared it to data from the Royal Greenwich Observatory in London, they found that observers at the Kodaikanal Observatory had underestimated the number of sunspots by about 40%. This discrepancy could have arisen, among other reasons, due to differences in observing conditions.

Also read: Why Indian Physicists Are Setting up a Tricky Experiment in an Active Uranium Mine

Fortunately, the authors of the January 9 preprint paper also reported that they were able to correct for the visual archive’s shortcomings, eliminating some errors, and allowing for the images’ continued use in scientific research.

Sakhee Bhure graduated with a BS in astronomy and astrophysics from the Florida Institute of Technology. She is interested in writing about science.

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