A Visit to Historic Yerkes Observatory

Note: I originally wrote this following a visit in 2009. In October 2018 – almost exactly 121 years after its dedication - Yerkes ceased research and closed to the public. It now faces an uncertain future. Consequently, the largest operational refractor in the world is now the 36” at Lick in California.

Any kid brought up on astronomy books and The Guinness Book of Records knows why Yerkes observatory is famous. Never mind the rococo architecture, nor the fact that many of the greatest astronomers of the 20th Century lived and worked there. No, the reason Yerkes is famous with schoolboys is that it is home to the largest operational refracting telescope on Earth. Now given my interest in all things refractor, I had to take the detour…

Getting There

Given its fame, Yerkes isn’t that easy to get to. I somehow expected it to be in the ‘burbs, but it’s not. In fact, heading northwest out of Chicago, Yerkes proved to be a two-hour drive.

On the way I could have popped in to legendary telescope and mount makers Astro-Physics, whose home is Machesney Park, on the way to Yerkes and just half an hour further west. Instead, I took the direct route through typical Midwestern suburbs, then light industrial and eventually endless undulating farmland prescient of the Great Plains just across the Mississippi.

Yerkes Observatory is situated on the north shore of Lake Geneva (sited presumably to take advantage of the stable seeing over the lake to its south), near the hamlet of Williams Bay in the state of Wisconsin.

I arrived via the only sizeable town on the lake - the eponymous one at its eastern end. From there you take the surprisingly hilly and remote-seeming highway 50 out of town, then take a left turn onto byway 67 to Williams Bay and on to Yerkes itself (a turning on the left past the village). The surrounding area is quite scenic and has an air of quiet affluence. Byway 67 runs past an inlet of the lake cluttered with marinas, shoreline walks and picnic areas.

The observatory is set well back from the road and was once surrounded by extensive (once 77 acre) lakefront parklands. Sadly, much of the original estate has been turned into a lakeside golf course and the observatory itself has already been saved from re-development once or twice.

Yerkes’ park-like setting, designed by a nephew of Frederick Olmsted.


Charles T. Yerkes was a Quaker entrepreneur with a dodgy reputation. He’d been imprisoned for larceny after the Chicago Fire and in an attempt to improve his image – and lobbied by George Ellery Hale, who was involved in so many of the big observatories of that era - Yerkes agreed with the University of Chicago to fund the world’s largest telescope. Only later was he persuaded to bankroll the complete Yerkes Observatory to the tune of $300,000.

The construction of Yerkes began – like the homes of other great refractors – in the late nineteenth century. Started in 1895, much of the main building and installation of the 40” was complete two years later. Observations began in summer 1897 and the observatory was dedicated in October.

Back then the Yerkes site looked very bare compared to the mature parkland of today, but the basis for the park was designed by John Charles Olmsted, nephew of the landscape architect Frederick Law Olmsted who designed Central Park (along with much else).

Yerkes hosted many of the most famous astronomers of the twentieth century, including Kuiper, Struve, Chandrasekhar and Hubble (who did his graduate work on faint nebulae there before moving to Mount Wilson). Yerkes has been called, ‘the birthplace of astrophysics’.

Yerkes had many famous visitors too. A picture of the Yerkes refractor in 1921 shows Albert Einstein among the assembled Yerkes staff. Latterly, Carl Sagan was a visitor.

The Observatory Buildings

Yerkes is the most beautiful observatory I have visited, both for its surrounding park, but also for its grand neoclassical architecture by Henry Cobb, ‘Beaux Arts’ details and elegant interior (where they were holding a wedding the day I was there).

The observatory is in the shape of a cross – a cathedral of science, perhaps. The long, flat-roofed main building, with numerous arched windows and a grand entrance porch, separates the big dome for the 40” refractor at one end and two smaller domes at the other which house 41” and 24” reflectors. The 41” reflector is a relatively modern Ritchey-Chretien, finished in the late 1960s and at one time operated robotically on Skynet.

Architectural details include whimsical mascarons and comic faces carved into its columns.

Unlike the functional dome bases at other observatories, those at Yerkes continue the neoclassical theme of the main building with arches, Corinthian columns and oculi, all in the same pink stone below the metal domes.

The interior of the main building continues the ornate carved stone of the entrance and has elegant rooms, including a wood-panelled library with squashy sofas and lots of astronomical globes – the kind of space I’d like for my own study.

Also of interest are displays on the construction of the 40” refractor and observatory.

Some elements of Yerkes’ Beaux Arts whimsical detailing.

Yerkes’ cosy library and study.

The 40” Clark Refractor

The Yerkes Great Refractor isn’t actually the largest ever made – that distinction went to a 48” (122cm) telescope built for the 1900 Great Exhibition in Paris. But the 40” is the largest ever mounted and used for astronomy.

Its objective lens, crafted in 1896, is an achromatic, air-spaced doublet weighing 225Kg with a focal length of 1930cm (F19 – same as the Lick 36”).

As for the 36” at Lick, the glass blanks came from France and were ground and polished by Alvan Clark and Sons in Massachusetts, like so many of the great refractors of the day. The Yerkes 40” would be the last big lens that Clark ground – he died in 1897.

Like its twin at Lick (and other big refractors of the era), the pier and mount were fabricated by Warner and Swasey. Before installation of the optics, telescope and mount were exhibited at the ‘World’s Columbian Exposition’, aka the ‘Chicago World’s Fair’, in 1893, now most famous for its pioneering use of electric light and power at the end of the ‘War of the Currents’ between Westinghouse and Edison.

The mount and tube look identical to Lick’s – a hugely long tube of gently tapering, riveted barrel-segments on a giant German equatorial atop a 40-foot high pier made of four cast iron sections. Unlike Lick, they have not retained their original finish but now sport a rather bright white and blue colour scheme.

The telescope would originally have had various wood-and-brass capstan wheels around the focuser, like those seen below on Lowell Observatories famous 24”. However, the 40” was still being used for research when I saw it and the visual back held a modern instrument package.

You enter the dome up a surprisingly long staircase from the ornate rotunda in the centre of the main building. The interior of the dome is a huge space, but a simple one of bare brick and iron, lacking even the wood panelling at Lick. The refractor and mount tower above, but seem to have more space than the tight-fitting dome for the 36” at Lick.

Like Lick, the floor of the dome can (or could at one time) be raised and lowered to accommodate the widely differing height of the telescope’s visual back when pointed at different altitudes. But the Yerkes floor appears to be operated by winches from above, rather than the fragile system of ratchets below the floor at Lick.

When I visited there was no public viewing program at Yerkes, though there had been one using the 1960s 24” reflector at one time (the 40” refractor itself was still being used for research). However, the main building houses a display of some early plates of the Moon taken with the 40” refractor by astronomers Frederick Slocum and George W. Ritchey (the co-inventor of the Ritchey Chretien optical system now employed in many professional reflectors).

Long flights of stairs up to the 40” dome.

Giant German mount for the 40”, built by warner and Swasey like the one at Lick.

Visual back of the restored 24” Clark refractor at the Lowell observatory – a sense of how the Yerkes Great Refractor would once have been.

Early plates of the Moon taken with the 40” Yerkes Clark refractor.

Since Yerkes observatory closed, I have visited both Lowell and Lick, where their great Clark refractors are available for public viewing. I hope a similar fate can be arranged for Yerkes.