As of this writing the most thorough Internet discussion of Sidney Coolidge's life is posted in an Internet Archive scan
of "Appletons' cylopedia of American biography", published in 1888. It seems reasonable to hope that the link provided to that scan will remain unbroken, but the provision for those with impaired vision feels awkward and inconvenient. For that reason, this page presents a transcript of the article in question:
scientist, born in Boston, Massachusetts, in 1830; died near Chickamauga, Georgia, 19 September, 1863. He studied abroad from 1839 till 1850, first in Geneva and Vevay, and afterward in the Royal military college in Dresden. After his return to this country he assisted in the construction of the Richmond and Danville railroad, and in running the boundary-line of Minnesota. After working in the nautical-almanac office and in the Cambridge observatory, he was appointed in 1853 assistant astronomer to Commodore Perry's Japan exploring expedition. In 1854 he assisted Professor George P. Bond in his observations of the planet Saturn, and contributed drawings and notes to the published annals of the observatory. He took charge in 1855 of the chronometric expedition for determining the difference of longitude between Cambridge and Greenwich, and in 1856 to 1857 studied the dialects and astronomical superstitions of the Indians near Saguenay river and Lake Mistassinnie. Being in Mexico in 1858, he took part in the civil war of that year, was taken prisoner and sentenced to be shot, but was finally released and sent to the city of Mexico on parole. He took part in an Arizona land-survey in 1860, and in May, 1861, became major in the 16th U.S. infantry. He was superintendent of the regimental recruiting service in 1862, commanded regiments at different posts and camps, and was engaged at the battles of Hoover's Gap and Chickamauga, where he was killed. For his services in the latter fight he received the brevet of lieutenant-colonel.
For those who reached this page from an external site,
here are additional notes from my NGC / IC Discoverers page
Phillip Sidney Coolidge (Aug 22, 1830 - Sep 19, 1863), American astronomical observer. A great-grandson of Thomas Jefferson through his daughter Martha, and her daughter Ellen Wayles Randolph. A major in the Union Army, he was one of four thousand men killed at Chickamauga. All of Coolidge's NGC objects proved to be stars, without any nebulosity. This was presumably due to the difficulty of distinguishing faint stars from equally faint, nearly stellar nebulosities, and the efforts of observers of the day (and today, for that matter) to exceed the limits of their instruments. Still, although his colleagues also made erroneous observations, Coolidge has the dubious distinction of being the least successful "discoverer" of NGC/IC objects (a sad footnote to an otherwise commendable career, though not as sad as its being cut so short). A discussion of Coolidge's friendship with William and George Bond, his observations of Saturn, and his loss at Chickamauga can be found here.