Question: Who was the emperor while Prince Sanjō Sanetomi was an Imperial court noble? 

Context articles: Fujiwara no Michimasa
Kujō Tsunenori
Meiji Restoration
Fujiwara no Narichika
Ichijō Sanetsune
Lady Sanjō
Prince Yamashiro
Kujō Tadaie
Sanjō Sanetomi
Fanzhen

was a mid-Heian period Imperial court noble and poet.
 He is included in the Hyakunin Isshu and was the nephew of Emperor Ichijo's wife, Empress Fujiwara no Teishi.
Kujō Tsunenori (九条 経教 , 1331 – 1400) , son of regent Michinori, was a "kugyō" or Japanese court noble of the Muromachi period (1336–1573).
 He held a regent position kampaku from 1358 to 1361.
 He married a daughter of Sanjō Sanetada, and the couple had sons Tadamoto, Noritsugu (教嗣 , ？-1404) and Mitsuie, among others.
The Meiji Restoration (明治維新 , Meiji Ishin ) , also known as the "Meiji Ishin", "Renovation", "Revolution", "Reform", or "Renewal", was an event that restored practical imperial rule to Japan in 1868 under Emperor Meiji.
 Although there were Emperors before the Meiji Restoration, the events restored practical abilities and consolidated the political system under the Emperor of Japan.
Fujiwara no Narichika (藤原 成親) (1138–1178) was a Japanese court noble who took part in a plot against the Taira clan's dominance of the Imperial court.
Ichijō Sanetsune (一条 実経 , 1223 – August 30, 1284) , son of regent Michiie, was a "kugyō" or Japanese court noble of the Kamakura period (1185–1333).
 He was the founding father of the Ichijō family, one of the five regent houses which monopolized regent positions in Japan's imperial court.
 He held regent positions kampaku in 1246 and from 1265 to 1267, and sessho from 1246 to 1247.
 Ietsune and Saneie were his sons.
Lady Sanjō (三条の方 , Sanjō no kata ) (1521 – August 29, 1570) was a Japanese woman of the Sengoku period.
 She was the wife of the famous daimyo, Takeda Shingen.
 She was the daughter of Sanjō Kinyori, a court noble of Kyoto; her sisters married Hosokawa Harumoto and Honganji Kennyo, respectively.
 Lady Sanjō married Shingen (then named Harunobu) at age 16.
 She gave birth to three sons and two daughters.
Prince Yamashiro (山背大兄王 , Yamashiro no Ōe no Ō , died 643) was the eldest son of one of the most famous figures in Japanese history, Prince Shōtoku.
 Yamashiro claimed the right to Imperial succession in 628, following the death of Empress Suiko.
 However, he lost the claim to Prince Tamura who ascended to the throne as Emperor Jomei, having enjoyed the support of powerful court noble Soga no Emishi.
Kujō Tadaie (九条 忠家 , 1229 – 1275) , son of regent Norizane, was a "kugyō" or Japanese court noble of the Kamakura period.
 He held regent positions kampaku from 1273 to 1274 and sessho in 1274.
 Tadanori and Kujō Tadatsugu (九条 忠嗣 , 1253-?)
 were his sons born by a daughter of Sanjō Kinfusa.
Prince Sanjō Sanetomi (三条 実美 , 13 March 1837 – 18 February 1891) was an Imperial court noble and statesman at the time of the Meiji Restoration.
 He held many high-ranking offices in the Meiji government.
Fanzhen (: "fānzhèn" ) was a governmental system involving administration through regional governors ("jiedushi)".
 The term "fanzhen" literally means "buffer town", and refers to the strategic settlement of troops in locations along the empire's border areas.
 During the Tang dynasty these settlements came under the control of provincial military commissioners, otherwise known as "jiedushi".
 As control of these "fanzhen" devolved from central authority into the hands of the local leaders, they at times became powerful enough to threaten the imperial court (618–907 CE), particularly during and after the An Lushan Rebellion.
 An Lushan, the provincial governor and military commander who started this rebellion against Emperor Xuanzong of Tang, in 755 CE, went so far as to proclaim himself emperor, in 756 CE, but, was killed by his own son in the following year and Tang power was re-established by 763 CE.
 The An Lushan Rebellion allowed many "jiedushi" on the periphery of the Tang Empire to gain significant autonomy with many becoming warlords in all but name.
 Subsequent Tang emperors were met with lukewarm success in curtailing the power of these "fanzhen", in particular, Emperor Dezong of Tang (r. 779–805 CE) who was driven from his capital, Chang'an, after an unsuccessful attempt to subjugate them.
 Subsequent Emperor Xianzong of Tang (r. 805–820 CE) was able to suppress some "fanzhen" but at the cost of further empowering the eunuchs who had come to dominate the life of the Imperial Court.
 Xianzong died in 820 CE, possibly as a result of assassination, and his successors were unable to stop the dynasty's decline.
 The ambitions of the "jiedushi", in tandem with the corruption of the Imperial Court eunuchs who dominated the central civil administration and even attained high military command under the later Tang, contributed to the disintegration of the Tang monarchy.
 A brief resurgence under emperors Wuzong and Xuānzong failed to halt the decentralization of state power, and the Tang collapsed following a further series of major peasant uprisings like the Wang Xianzhi and Huang Chao rebellions.