Candidate Observatory Site:
Yerkes Observatory, Williams Bay, WI


42 34.22 N, 88 33.37 W
(Copy and paste this location into or for images and maps of site.)


334 meters (1096 ft)

Site Description

The site lies on a flat-to-rolling glacial plain dotted with small towns. It is an improved site with a functioning observatory and excellent infrastructure. Roads exist right up to the site, so no new rods need be constructed. Nearby world-class institutions include the University of Wisconsin at Madison and the University of Chicago (Yerkes parent institution), both with outstanding departments of astronomy and space science. The nearest major centers of population are Milwaukee (47 miles or 76 km, population 603,000) Madison (55 miles or 88 km, population 224,000), and Chicago (67 miles or 108 km, population ~3 million), guaranteeing plentiful skilled construction and support workers. The site for Yerkes was originally picked (~1895) for outstanding accessibility and infrastructure support, dark skies, distance from lake-effect snow around Lake Michigan, and to be upwind from the coal soot and pollution of metropolitan Chicago.

Site Images

aerial view of yerkes
Representative view of Yerkes showing the existing observatory, typical vegetation and terrain surrounding the site. University of Chicago image.

41 inch dome at yerkes
View of the 41-inch telescope dome, the principal research instrument at Yerkes. Lake Genevba is visible in the background. University of Chicago image.


The best observing is in July or August, particular during dry years, declining during the Fall into the worst weather in December and January, then slowly improving again into the spring and early summer.

Site Testing Information


Typical; seeing at Yerkes is about 2" of arc. The telescope is not used when the seeing becomes poorer than about 5". Seeing can reach 1 – 1.5", typically when hig pressure builds in after the passage of a front.

yerkes night seeing
Representative seeing on a typical night at Yerkes, starting with clear steady air, with intervals of turbulence and haze passing over after midnight..

Clear Nights.

Yerkes averages about 100 clear nights per year (~50 in poor years, ~150 in good years)

Precipitable Water.

(See humidity measurements on weather site below.)

Additional Information.

Current and seasonal weather conditions may be found at the National Weather Service or for Williams Bay. For additional information about the site, visit the Yerkes web page.