National Astronomical Observatory of Japan



Solar Observations Begin Second Century


Solar Observations Begin Second Century

The left image is a monochromatic photograph of the Sun taken in the calcium K line (hereinafter, called the “Ca K-line”). It is a digitized photograph taken 100 years ago with a spectroheliograph. At that time, solar observations were performed at Azabu in Tokyo. Regular full-disk solar observation began from this time. And now, 100 years after the first solar observation, we observe the Sun in the Ca K-line using the Solar Flare Telescope in NAOJ Mitaka Campus (right photo).

Solar Ca K-Line Images Taken Recently and 100 Years Ago

The left photo is a monochromatic image of the Sun taken in the Ca K-line 100 years ago on January 20, 1917. The regular full-disk solar observations of NAOJ started from this day (Note). This image of the Sun was taken with a spectroheliograph which performed solar observations at Azabu in Tokyo at that time. With the Ca K-line, we can observe the chromosphere, which is above the solar surface that can usually be seen. The bright white parts are called “plages,” and correspond to active regions. Ca K-line (blue-violet, 393.4 nanometers) is the widest absorption line in the visible light. This wavelength can easily be observed with even a relatively simple spectrograph. Therefore, solar observations at this wavelength were conducted for a long time: 1917 to 1924 in Azabu and from 1925 to 1974 in Mitaka. These observational data are valuable clues to understand the solar activity at that time. After that, the Ca K-line solar observations stopped temporarily. And in July 2015, we restarted observations using the continuum/ G-band/ Ca K-line full-disk imager of the Solar Flare Telescope. This year, 2017, is the 100th anniversary of NAOJ’s solar observations (right photo).

Text by: Kentaro Yaji (Solar Science Observatory, NAOJ)
Translation by: Hiroko Tsuzuki and Ramsey Lundock (Public Relations Center, NAOJ)

(Note) It was known as Tokyo Astronomical Observatory at that time. Back

Image Data

ObjectThe Sun
TelescopeGrubb 30-centimeter siderostat and Cooke 18-centimeter objective lens
Wavelength393.4 nanometers
DateJanuary 20, 1917
CreditNational Astronomical Observatory of Japan
Left Image Data
ObjectThe Sun
TelescopeSolar Flare Telescope
InstrumentContinuum/ G-band/ Ca K-line Full-disk Imager
Wavelength393.4 nanometers
Exposure20 milliseconds
DateJanuary 21, 2017
CreditNational Astronomical Observatory of Japan
Right Image Data


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