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Look at a chart of where sunspots appear over the years and you see a row of butterfly wings: spots start in two bands at mid-latitudes, then creep towards the Sun's equator, and the next cycle starts high again. That pattern was charted in 1904 from Greenwich photographs of the Sun, the kind Annie Russell Maunder spent years measuring. This post covers who she was, what she did with her own hands and camera, how to read a sunspot's latitude from a photograph, and what her career teaches.
Who was Annie Maunder?
The Dictionary of Irish Biography gives her birth as 14 April 1868 in Strabane, County Tyrone, the daughter of a Presbyterian minister. She won a scholarship to Girton College, Cambridge, and was ranked senior optime in the mathematical tripos in 1889, though women could not then receive Cambridge degrees. After a year teaching mathematics in Jersey, she joined the Royal Observatory, Greenwich, in 1891.
Her obituary in Monthly Notices of the Royal Astronomical Society, written by fellow astronomer M. A. Evershed, says she was one of four young women appointed "Lady Computers" that year, and that part of her work was "to examine and measure the daily sunspot photographs". The DIB adds that she photographed the Sun, developed the plates and measured them with a micrometer. She died in London on 15 September 1947.
Why did she leave Greenwich, and what did she keep doing?
In December 1895 she married Edward Walter Maunder, head of the observatory's Photographic and Spectroscopic Department, and had to resign under civil service rules. She kept working as an unpaid researcher. The obituary describes "an ideal partnership" in which they wrote many articles together, signed with both names.
The Royal Astronomical Society refused her as a fellow because she was a woman, the DIB records, though in 1892 she was among the very few women invited to attend its meetings. She was elected a fellow on 10 November 1916, after the ban was lifted. She also edited the journal of the British Astronomical Association for many years.
What did she photograph in India in 1898?
The couple went to five total solar eclipses together. At the one in India in 1898, the obituary says, "she secured a photograph of a longer coronal streamer than had ever been photographed before", with an instrument she had prepared for that purpose. The DIB says she developed the camera to her own specifications.
There is another Indian link. Her 1907 paper notes that the Greenwich solar record of 1889 to 1901 used photographs of the Sun taken daily "at Greenwich, in India, and in Mauritius"; a 1902 Greenwich report names the Indian station as Dehra Dun.
What is the butterfly diagram, and who made it?
The 1904 paper in Monthly Notices that presented the diagrams is signed by Walter Maunder. It summarises Greenwich measurements from 1874 to 1902: about 9,000 photographs, about 5,000 spot groups, and "upwards of a quarter of a million measures made in duplicate". The DIB credits the chart to the husband-and-wife team. What the sources do show is that measuring such photographs had been Annie's job at Greenwich, and that the couple then worked on the Sun together for decades.
NASA's Marshall Space Flight Center describes what such a diagram shows: spots are concentrated in two bands either side of the equator that "first form at mid-latitudes, widen, and then move toward the equator as each cycle progresses", with a cycle of about 11 years.
Her own major paper, An Apparent Influence of the Earth on the Numbers and Areas of Sun-spots in the Cycle 1889-1901 (1907), used the Greenwich spot "ledgers" to test whether spot numbers depend on where the Earth is. The obituary says she found fewer spots as they passed from the east to the west half of the disc as seen from Earth, a result that by 1948 had "never since been disproved or satisfactorily assigned to any other cause".
Worked example: a sunspot's latitude from a photograph
Each photograph had to be turned into a position on the Sun. Here is the simplest case, ignoring the small tilt of the Sun's axis as seen from Earth.
Set up. The Sun's image has radius R. A spot sits on the central meridian, a distance y north of the disc's centre.
Geometry. The spot is on a sphere of radius R, so y = R sin φ, where φ is its latitude. So sin φ = y/R.
Numbers (sample). On a photograph with R = 100 mm, a spot 50 mm north of centre gives sin φ = 0.50, so φ = 30° N.
Through a cycle. At y = 57.4 mm, φ = 35°; at y = 25.9 mm, φ = 15°. A butterfly diagram plots φ against time, so a cycle's spots slide down from around 30° towards the equator.
Scale of the work. A quarter of a million measures on 9,000 photographs is about 28 measures per photograph, each done twice.
Common mistake: using y/R as the angle itself. The ratio is a sine, and at large latitudes the difference matters. India now watches the Sun from space too; our post on Aditya-L1 covers that mission.
If you want help turning a geometry set-up like this into steps, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning.
What can a student take from how she worked?
Routine measurement is real science. The 1904 diagrams rested on about 9,000 photographs, every measure made twice.
Ask a sharp question of old data. Her 1907 paper used records already published, and asked something new of them.
Build the tool you need. For the 1898 eclipse she equipped her own camera for one goal, and got the record.
Keep going when the rules shut you out. She lost her post on marriage and was refused fellowship, and kept publishing.
Next step: with R = 100 mm, find the latitude of spots 10, 40 and 70 mm from the centre on the central meridian, and sketch where they would sit on a butterfly diagram.
