Career
In 1877 he observed Mars and created a detailed map of its surface features. He organized an expedition to Santiago de Chile to observe the Venus transit the Sun in 1882. A second Belgian expedition (led by Jean-Charles Houzeau) travelled to San Antonio, Texas to determine the planet"s parallax during the same transit.
Both parties used a specially designed heliometer for this purpose.
A crater on Mars was named in his honor.