A central theme of “Planetarium” is investigating the experience of being a woman working as a scientist, specifically an astronomer. Rich contrasts the historical figure of Caroline Herschel, who the poem’s epigraph (dedication) mentions with the historical figure of Tycho Brahe, who is named in Line 24. Facts about Herschel’s career, such as the number of comets she discovered—“8 comets” (Line 8)—can be contrasted with facts about Brahe, such as the name of his Danish astronomical observatory: “Uranusborg” (Line 19). Even before his name is included, Rich alludes to Brahe by including the name of his observatory. Readers are more likely to be familiar with Brahe than Herschel, so Rich includes her name before the poem and lists specific discoveries to help increase knowledge about her work.

Brahe’s own words are also included in the poem. Rich includes three sets of quotes in Stanzas 2, 7, and 11. Stanza 2 and Stanza 7 are about Herschel and Brahe, respectively, but neither includes a quote from an English source. Either Rich created the encyclopedia-like quotes herself, or she translated them from another language. In contrast to this, in Stanza 11, Rich includes Brahe’s famous quote “Let me not seem to have lived in vain” (Line 25), and she clearly cites him.