The week that Mars moved to its closest point to Earth in 60,000 years, one of the area’s best-kept secrets was revealed. About 400 people thronged to Merrimack College in North Andover on Aug. 27 to view Mars through the high-powered telescope housed in the school’s observatory. Ordinarily, just a handful of stargazers show up at the observatory on Wednesday evenings, when the domed room is open to the public.