“No Sun Spots Visible”

“No Sun Spots Visible”

―John Edwards, 1913.

In February 1907 John Edwards, assistant to astronomy professor Winslow Upton, recorded a drawing of spots that he observed on the Sun. Due to the brightness of the Sun it is not safe to look at it through a telescope. Instead he used a method called eyepiece projection which forms an image on a sheet of circular graph paper. The outline of the spots can then be accurately traced with a pencil. The sketch shows a complex arrangement of sunspots during a month when the Sun was very active. This was shortly after the peak of the Sun’s 11 year cycle of increasing and then decreasing activity. Sunspots appear in active regions where there are strong and complex magnetic fields.

Sunspot drawing
Sunspot observations, February 11, 1907 at 9:55 am.

By 1913 the Sun’s cycle had reached the minimum of the cycle and no spots were visible during some months that year. It is important to note the lack of sunspot activity so that someone examining the preserved records a century later knows that the astronomer looked and didn’t find any. Otherwise it might be thought that the data was merely missing. Notice that on Jul 18, 1913 there is a note that it was cloudy that day.

Record of no Sunspots visible
No sun spots visible, July 13 to August 15, 1913.

The magnitude of the peak of each cycle can vary. The cycle in the late 1950s was one of strongest in two centuries. The most recent in 2012-2014 was one of the weakest in a century.

Graph of sunspot activity
Prior sunspot cycles and prediction (in red) of the next peak.

Today we use solar telescope with special filters. The hydrogen alpha filter only allows a single color of red light to pass through it. This is tuned to the color of light emitted by the most abundant chemical element found in stars – hydrogen. In addition to sunspots it also reveals rich detail in a layer of the Sun called the chromosphere. Bright prominences can be seen near the edge of the Sun.

Hydrogen alpha telescope image of the Sun
April 2, 2019

A more common type of protective solar filter is called white light. This blocks harmful ultraviolet light and reduced the overall brightness to a safe level. This scaptures light that originates in a layer of the Sun below the chromosphere called the photosphere.

White light telescope image of the Sun
April 18, 2022

These two recent images were taken by Brown University students enrolled in the introductory astronomy class using telescopes from Ladd Observatory. The image processing was done by Scott Macneill.

Solar activity is just beginning to increase after the last minimum and is predicted to peak in 2025.

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