Christian was only nominally king from 1772 onward. Between 1772 and 1784, Denmark-Norway was ruled by his stepmother, the Queen Dowager Juliane Marie, his half-brother Frederick, and the Danish politician Ove Høegh-Guldberg. From 1784, his son Frederick VI ruled permanently as prince regent.
On the recommendation of George Frederick, Margrave of Brandenburg-Ansbach, he was admitted to the Margrave's Academy in Heilsbronn in 1586, where he studied until 1601. During this time, he published observations about a comet as well as astronomical tables, which gave him a reputation as a good astronomer and mathematician, and the Margrave appointed him as his official mathematician. 
Albert II, Margrave of Brandenburg-Ansbach: 6. John Frederick, Margrave of Brandenburg-Ansbach: 13. Countess Sophie Margarete of Oettingen-Oettingen: 3. Princess Caroline of Brandenburg-Ansbach: 14. John George I, Duke of Saxe-Eisenach: 7. Princess Eleonore Erdmuthe of Saxe-Eisenach: 15. Johannetta, Countess of Sayn-Altenkirchen
William Frederick, Prince of Nassau-Dietz: 8. Henry Casimir II, Count of Nassau-Dietz: 17. Countess Albertine Agnes of Nassau: 4. John William Friso, Prince of Orange: 18. John George II, Prince of Anhalt-Dessau: 9. Henriëtte Amalia van Anhalt-Dessau: 19. Henriette Catherine of Nassau: 2. William IV, Prince of Orange: 20. William VI, Landgrave ...
John George was married twice. In addition to his successor John George II, he left three sons, Augustus (1614–1680), Christian (died 1691) and Maurice (died 1681). In Dresden on 16 September 1604 Johann Georg married firstly Sibylle Elisabeth, daughter of Frederick I, Duke of Württemberg. She died in the birth of their only child:
John Frederick, Margrave of Brandenburg-Ansbach (father of 187) 375. Princess Eleonore Erdmuthe of Saxe-Eisenach (mother of 187) 376. William Henry, Prince of Nassau-Usingen (father of 188) 377. Charlotte Amalie, Countess of Nassau-Dillenburg (mother of 188) 378. John William III, Duke of Saxe-Eisenach (father of 189) 379.
Daughter of John Frederick, Margrave of Brandenburg-Ansbach and Princess Eleonore Erdmuthe of Saxe-Eisenach: 22 August 1705 11 June 1727 Spouse's accession: 11 October 1727 20 November 1737 54 years, 172 days Westminster Abbey: 10 years, 162 days George II: Princess Charlotte of Mecklenburg-Strelitz: 19 May 1744