684 Hildburg (prov. designation: or ) is a stony background asteroid from the inner regions of the asteroid belt. It was discovered on 8 August 1909, by German astronomer August Kopff at the Heidelberg-Königstuhl State Observatory in southwest Germany. The S-type asteroid has a rotation period of 15.9 hours and measures approximately in diameter. Any reference to the origin of the asteroid's name is unknown. Orbit and classification Hildburg is a non-family asteroid of the main belt's background population when applying the hierarchical clustering method to its proper orbital elements. It orbits the Sun in the inner main-belt at a distance of 2.3–2.5 AU once every 3 years and 9 months (1,385 days; semi-major axis of 2.43 AU). Its orbit has an eccentricity of 0.04 and an inclination of 6° with respect to the ecliptic. The body's observation arc begins at Heidelberg Observatory on 19 August 1909, just eleven nights after its official discovery observation. Naming This minor planet's name may have been inspired by the two letters of its provisional designation, "1909 HD". However, any reference to a person or occurrence is unknown. It is speculated that the name comes from a list created in 1913 by the Astronomisches Rechen-Institut (ARI) containing suggestions of female names from history and mythology for the naming of minor planets (AN 196, 137). At the time, the naming process was not well developed and the ARI feared inconsistencies and potential confusion. The list was sent to several German astronomers, including Kopff, with the invitation to name all of their made discoveries up to number 700. It is therefore unlikely that this asteroid's name refers to a known person such as Austrian actress (1862–1942). Unknown meaning Among the many thousands of named minor planets, Hildburg is one of 120 asteroids for which has been published. All of these asteroids with an unknown meaning have low numbers, beginning with and ending with , all discovered between 1876 and the 1930s, predominantly by astronomers Auguste Charlois, Johann Palisa, Max Wolf and Karl Reinmuth. Physical characteristics Hildburg is a common stony S-type asteroid according to observations by Richard Binzel conducted at McDonald and Cerro Tololo observatories in May 1984. Rotation period In March 2014, a rotational lightcurve of Hildburg was obtained from photometric observations by Italian astronomer Andrea Ferrero at the Bigmuskie Observatory in Mombercelli, Italy. Lightcurve analysis gave a rotation period of hours with a brightness