Sol Invictus ("Unconquered Sun") was the official sun god of the later Roman Empire and a patron of soldiers. In 274 the Roman emperor Aurelian made it an official cult alongside the traditional Roman cults. Scholars disagree whether the new deity was a refoundation of the ancient Latin cult of Sol, a revival of the cult of Elagabalus or completely new. The god was favored by emperors after Aurelian and appeared on their coins until Constantine I. The last inscription referring to Sol Invictus dates to AD 387, and there were enough devotees in the 5th century that Augustine found it necessary to preach against them.
The idea, particularly popular in the 18th and 19th centuries, that the solstice date of 25 December for Christmas was selected in because it was also the date of a Roman festival of Dies Natalis Solis Invicti (Birthday of the Unconquered Sun) is challenged by a different explanation considered academically to be "thoroughly viable". Both theories have supporters, with some claiming that the festival of Dies Natalis Solis Invicti was later syncretized with Christmas,[need quotation to verify] and others saying that the Christian celebration may even predate the festival of the Dies Natalis Solis Invicti.
Invictus as epithet
Invictus ("Unconquered, Invincible") was an epithet for several deities of classical Roman religion, including the supreme deity Jupiter, the war god Mars, Hercules, Apollo, and Silvanus. It had been in use from the 3rd century BC. The Roman cult to Sol is continuous from the "earliest history" of the city until the institution of Christianity as the exclusive state religion. Scholars have sometimes regarded the traditional Sol and Sol Invictus as two separate deities, but the rejection of this view by S.E. Hijmans has found supporters.
An inscription of AD 102 records a restoration of a portico of Sol in what is now the Trastevere area of Rome by a certain Gaius Iulius Anicetus. While he may perhaps have had in mind an allusion to his own cognomen, which is the Latinized form of the Greek equivalent of invictus, ἀνίκητος (aniketos), the earliest extant dated inscription that uses invictus as an epithet of Sol is of AD 158. Another, stylistically dated to the 2nd century, is inscribed on a Roman phalera (ornamental disk): inventori lucis soli invicto augusto ("to the contriver of light, sol invictus augustus"). Augustus is a regular epithet linking deities to the Imperial cult.
Sol Invictus played a prominent role in the Mithraic mysteries, and was equated with Mithras himself. The relation of the Mithraic Sol Invictus to the public cult of the deity with the same name is unclear and perhaps non-existent.
The first sun god consistently termed invictus was the provincial Syrian god Elagabalus. According to the Historia Augusta, the teenaged Severan heir adopted the name of his deity and brought his cult image from Emesa to Rome. Once installed as emperor, he neglected Rome's traditional State deities and promoted his own as Rome's most powerful deity. This ended with his murder in 222. The Historia Augusta refers to the deity Elagabalus as "also called Jupiter and Sol" (fuit autem Heliogabali vel Iovis vel Solis). While this has been seen as an attempt to import the Syrian sun god to Rome, the Roman cult of Sol had existed in Rome in the earlier Republic.
The Roman gens Aurelian was associated with the cult of Sol. After his victories in the East, the Emperor Aurelian thoroughly reformed the Roman cult of Sol, elevating the sun-god to one of the premier divinities of the Empire. Where previously priests of Sol had been simply sacerdotes and tended to belong to lower ranks of Roman society, they were now pontifices and members of the new college of pontifices instituted by Aurelian. Every pontifex of Sol was a member of the senatorial elite, indicating that the priesthood of Sol was now highly prestigious. Almost all these senators held other priesthoods as well, however, and some of these other priesthoods take precedence in the inscriptions in which they are listed, suggesting that they were considered more prestigious than the priesthood of Sol. Aurelian also built a new temple for Sol, bringing the total number of temples for the god in Rome to (at least) four He also instituted games in honor of the sun god, held every four years from AD 274 onwards.
The identity of Aurelian's Sol Invictus has long been a subject of scholarly debate. Based on the Historia Augusta, some scholars have argued that it was based on Sol Elagablus (or Elagabla) of Emesa. Others, basing their argument on Zosimus, suggest that it was based on the Helios, the solar god of Palmyra on the grounds that Aurelian placed and consecrated a cult statue of Helios looted from Palmyra in the temple of Sol Invictus. Professor Gary Forsythe discusses these arguments and adds a third more recent one based on the work of Steven Hijmans. Hijmans argues that Aurelian's solar deity was simply the traditional Greco-Roman Sol Invictus.
Emperors portrayed Sol Invictus on their official coinage, with a wide range of legends, only a few of which incorporated the epithet invictus, such as the legend SOLI INVICTO COMITI, claiming the Unconquered Sun as a companion to the Emperor, used with particular frequency by Constantine. Statuettes of Sol Invictus, carried by the standard-bearers, appear in three places in reliefs on the Arch of Constantine. Constantine's official coinage continues to bear images of Sol until 325/6. A solidus of Constantine as well as a gold medallion from his reign depict the Emperor's bust in profile twinned (jugate) with Sol Invictus, with the legend INVICTUS CONSTANTINUS
Constantine decreed (March 7, 321) dies Solis—day of the sun, "Sunday"—as the Roman day of rest [CJ3.12.2]:
- On the venerable day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed. In the country however persons engaged in agriculture may freely and lawfully continue their pursuits because it often happens that another day is not suitable for grain-sowing or vine planting; lest by neglecting the proper moment for such operations the bounty of heaven should be lost.
Constantine's triumphal arch was carefully positioned to align with the colossal statue of Sol by the Colosseum, so that Sol formed the dominant backdrop when seen from the direction of the main approach towards the arch.
Sol and the other Roman Emperors
Berrens deals with coin-evidence of Imperial connection to the Solar cult. Sol is depicted sporadically on imperial coins in the 1st and 2nd centuries AD, then more frequently from Septimius Severus onwards until AD 325/6. Sol invictus appears on coin legends from AD 261, well before the reign of Aurelian.
Connections between the imperial radiate crown and the cult of Sol are postulated. Augustus was posthumously depicted with radiate crown, as were living emperors from Nero (after AD 65) to Constantine. Some modern scholarship interprets the imperial radiate crown as a divine, solar association rather than an overt symbol of Sol; Bergmann calls it a pseudo-object designed to disguise the divine and solar connotations that would otherwise be politically controversial but there is broad agreement that coin-images showing the imperial radiate crown are stylistically distinct from those of the solar crown of rays; the imperial radiate crown is depicted as a real object rather than as symbolic light. Hijmans argues that the Imperial radiate crown represents the honorary wreath awarded to Augustus, perhaps posthumously, to commemorate his victory at the battle of Actium; he points out that henceforth, living emperors were depicted with radiate crowns, but state divi were not. To Hijmans this implies the radiate crown of living emperors as a link to Augustus. His successors automatically inherited (or sometimes acquired) the same offices and honours due to Octavian as "saviour of the Republic" through his victory at Actium, piously attributed to Apollo-Helios. Wreaths awarded to victors at the Actian Games were radiate.
Sol Invictus and Christianity and Judaism
The idea that Christians chose to celebrate the birth of Jesus on 25 December because this was the date of an already existing festival of the Sol Invictus was expressed in an annotation to a manuscript of a work by 12th-century Syrian bishop Jacob Bar-Salibi. The scribe who added it wrote: "It was a custom of the Pagans to celebrate on the same 25 December the birthday of the Sun, at which they kindled lights in token of festivity. In these solemnities and revelries the Christians also took part. Accordingly when the doctors of the Church perceived that the Christians had a leaning to this festival, they took counsel and resolved that the true Nativity should be solemnised on that day." 
In the judgement of the Church of England Liturgical Commission, this view has been seriously challenged by a view based on an old tradition, according to which the date of Christmas was fixed at nine months after 25 March, the date of the vernal equinox, on which the Annunciation was celebrated. The Jewish calendar date of 14 Nisan was believed to be that of creation, as well as of the Exodus and so of Passover, and Christians held that the new creation, both the death of Jesus and the beginning of his human life, occurred on the same date, which some put at 25 March in the Julian calendar. It was a traditional Jewish belief that great men lived a whole number of years, without fractions, so that Jesus was considered to have been conceived on 25 March, as he died on 25 March, which was calculated to have coincided with 14 Nisan. Sextus Julius Africanus (c.160 – c.240) gave 25 March as the day of creation and of the conception of Jesus. The tractate De solstitia et aequinoctia conceptionis et nativitatis Domini nostri Iesu Christi et Iohannis Baptistae falsely attributed to John Chrysostom also argued that Jesus was conceived and crucified on the same day of the year and calculated this as 25 March. A passage of the Commentary on the prophet Daniel by Hippolytus of Rome, written in about 204, has also been appealed to.
Among scholars who view the celebration of the birth of Jesus on 25 December as motivated by choice of the winter solstice, rather than that he was conceived and died on 25 March, some reject the idea that this choice constituted a deliberate Christianization of a festival of the Birthday of the Unconquered Sun. For example, Michael Alan Anderson writes:
Both the sun and Christ were said to be born anew on December 25. But while the solar associations with the birth of Christ created powerful metaphors, the surviving evidence does not support such a direct association with the Roman solar festivals. The earliest documentary evidence for the feast of Christmas makes no mention of the coincidence with the winter solstice. Thomas Talley has shown that, although the Emperor Aurelian's dedication of a temple to the sun god in the Campus Martius (C.E. 274) probably took place on the 'Birthday of the Invincible Sun' on December 25, the cult of the sun in pagan Rome ironically did not celebrate the winter solstice nor any of the other quarter-tense days, as one might expect. The origins of Christmas, then, may not be expressly rooted in the Roman festival.
The same point is made by Hijmans: "It is cosmic symbolism...which inspired the Church leadership in Rome to elect the southern solstice, December 25, as the birthday of Christ ... While they were aware that pagans called this day the 'birthday' of Sol Invictus, this did not concern them and it did not play any role in their choice of date for Christmas." He also states that, "while the winter solstice on or around December 25 was well established in the Roman imperial calendar, there is no evidence that a religious celebration of Sol on that day antedated the celebration of Christmas".
The Oxford Companion to Christian Thought also remarks on the uncertainty about the order of precedence between the celebrations of the Birthday of the Unconquered Sun and the birthday of Jesus: "This 'calculations' hypothesis potentially establishes 25 December as a Christian festival before Aurelian's decree, which, when promulgated, might have provided for the Christian feast both opportunity and challenge."
Susan K. Roll also calls "most extreme" the unproven hypothesis that "would call Christmas point-blank a 'christianization' of Natalis Solis Invicti, a direct conscious appropriation of the pre-Christian feast, arbitrarily placed on the same calendar date, assimilating and adapting some of its cosmic symbolism and abruptly usurping any lingering habitual loyalty that newly-converted Christians might feel to the feasts of the state gods".
The comparison of Christ with the astronomical Sun is common in ancient Christian writings. In the 5th century, Pope Leo I (the Great) spoke in several sermons on the Feast of the Nativity of how the celebration of Christ's birth coincided with increase of the sun's position in the sky. An example is: "But this Nativity which is to be adored in heaven and on earth is suggested to us by no day more than this when, with the early light still shedding its rays on nature, there is borne in upon our senses the brightness of this wondrous mystery.
A study of Augustine of Hippo remarks that his exhortation in a Christmas sermon, "Let us celebrate this day as a feast not for the sake of this sun, which is beheld by believers as much as by ourselves, but for the sake of him who created the sun", shows that he was aware of the coincidence of the celebration of Christmas and the Birthday of the Unconquered Sun, although this pagan festival was celebrated at only a few places and was originally a peculiarity of the Roman city calendar. It adds: "He also believes, however, that there is a reliable tradition which gives 25 December as the actual date of the birth of our Lord."
By "the sun of righteousness" in Malachi 4:2 "the fathers, from Justin downward, and nearly all the earlier commentators understand Christ, who is supposed to be described as the rising sun". The New Testament itself contains a hymn fragment: "Awake, O sleeper, and arise from the dead, and Christ will shine on you." Clement of Alexandria wrote of "the Sun of the Resurrection, he who was born before the dawn, whose beams give light".
Christians adopted the image of the Sun (Helios or Sol Invictus) to represent Christ. In this portrayal he is a beardless figure with a flowing cloak in a chariot drawn by four white horses, as in the mosaic in Mausoleum M discovered under Saint Peter's Basilica and in an early-4th-century catacomb fresco. Clement of Alexandria had spoken of Christ driving his chariot in this way across the sky. The nimbus of the figure under Saint Peter's Basilica is described by some as rayed, as in traditional pre-Christian representations, but another has said: "Only the cross-shaped nimbus makes the Christian significance apparent" (emphasis added). Yet another has interpreted the figure as a representation of the sun with no explicit religious reference whatever, pagan or Christian.
The traditional image of the sun is used also in Jewish art. A mosaic floor in Hamat Tiberias presents David as Helios surrounded by a ring with the signs of the zodiac. As well as in Hamat Tiberias, figures of Helios or Sol Invictus also appear in several of the very few surviving schemes of decoration surviving from Late Antique synagogues, including Beth Alpha, Husefah (Husefa), all now in Israel, and Naaran in the West Bank. He is shown in floor mosaics, with the usual radiate halo, and sometimes in a quadriga, in the central roundel of a circular representation of the zodiac or the seasons. These combinations "may have represented to an agricultural Jewish community the perpetuation of the annual cycle of the universe or ... the central part of a calendar".
- See S.E.Hijmans, "The sun that did not rise in the east", Babesch 71 (1996) p.115–150
- See Gaston Halsberghe, "The cult of Sol Invictus", Leiden: Brill, 1972
- As Hijmans states (p.115): "Scholars have consistently postulated a clear distinction between the Republican Sol Indiges and the Imperial Sol Invictus." and p.116 "We should keep in mind, however, that most scholars agree that this cult [Sol Indiges] was never important, and that it had disappeared altogether by the beginning of the second century AD"
- Halsberghe, "The cult of Sol Invictus", p.155: "Up to the conversion of Constantine the Great, the cult of Deus Sol Invictus received the full support of the emperors. The many coins showing the sun god that these emperors struck provide official evidence of this." and p.169 "the custom of representing Deus Sol Invictus on coins came to an end in AD 323."
- Halsberghe, "The cult of Sol Invictus", p.170 n.3: "CIL VI, 1778, dates from AD 387,"
- Halsberghe, p.170, n.4: "Augustine, Sermones, XII; also in Ennaratio in Psalmum XXV; Ennaratio II, 3."
- "Although this view is still very common, it has been seriously challenged" - Church of England Liturgical Commission, The Promise of His Glory: Services and Prayers for the Season from All Saints to Candlemas" (Church House Publishing 1991 ISBN 978-0-71513738-3) quoted in The Date of Christmas and Epiphany
- Susan K. Roll, Toward the Origin of Christmas (Peeters Publications 1995 ISBN 978-90-3900531-6), p. 88
- Intransigence and Indifference: Essays Concerning Religion and Spirituality (2009) by Jarred James Breaux
- Steven Hijmans, "Sol Invictus, the Winter Solstice, and the Origins of Christmas" in Mouseion, Number 47/3 (2003), pp. 384–385
- Michael Alan Anderson, Symbols of Saints (ProQuest 2008 ISBN 978-0-54956551-2), p. 46
- Karen B. Westerfield Tucker, "Christmas" in Adrian Hastings, Alistair Mason, Hugh Pyper (editors), The Oxford Companion to Christian Thought (Oxford University Press 2000 ISBN 978-0-19860024-4), p. 124
- CIL VI.31181.
- Hijmans, S. E. (2009). "The sun which did not rise in the east", p. 124.
- Hijmans, Steven Ernst. (2009). Sol: The Sun in the Art and Religions of Rome (diss., University of Groningen 2009), p. 18, with citations from the Corpus Inscriptionum Latinarum.
- Hijmans (2009, pp. Chapter 1) (a reworking of Hijmans, 1996; Matern, 2001; Wallraff, 2002; and Berrens, 2004; all follow Hijmans.
- (Hijmans 2009, pp. 483–508 (chapter 5))
- (Hijmans, 2009, 486, footnote 22)
- CIL VI, 715: Soli Invicto deo / ex voto suscepto / accepta missione / honesta ex nume/ro eq(uitum) sing(ularium) Aug(usti) P(ublius) / Aelius Amandus / d(e)d(icavit) Tertullo et / Sacerdoti co(n)s(ulibus) (Publius Aelius Amandus dedicated this to the god Sol Invictus in accordance with the vow he had made, upon his honorable discharge from the equestrian guard of the emperor, during the consulship of Tertullus and Sacerdos); see: Campbell, J. (1994). The Roman army, 31 BC–AD 337: a sourcebook, p. 43; Halsberghe 1972, p. 45.
- Guarducci, M. (1957/1959). Sol invictus augustus. Rendiconti della Pont. Accademia Romana di archeologia, 3rd series 30/31 pp. 161 ff. An illustration is provided in Kantorowicz, E. H. (1961). Gods in Uniform. Proceedings of the American Philosophical Society, 105(4), 368–393, 383, fig. 34 (August 1961).
- David Ulansey, The Origins of the Mithraic Mysteries (Oxford University Press, 1989), p. 107; Salzman, Michele Renee. (2004). Pagan and Christian Notions of the Week in the 4th Century CE Western Roman Empire In Time and Temporality in the Ancient World. p. 192, University of Pennsylvania Museum of Archaeology and Anthropology; Alvar, Jaime, tr. Gordon, Richard (2008). Romanising Oriental Gods: Myth, Salvation and Ethics in the Cults of Cybele, Isis and Mithras, p. 100. Brill.
- (Alvar, 2008, p. 203)
- Historia Augusta, 1, 5: English translation (Loeb) from Thayer  & Latin text 
- See in particular Halsberghe 1972.
- Hijmans 1996, Matern 2001, Wallraff 2002, Berrens 2004, Hijmans (2009)).
- J.C. Richard, “Le culte de Sol et les Aurelii. A propos de Paul Fest. p. 22 L.”, in: Mélanges offerts à Jacques Heurgon. L'Italie préromaine et la Rome républicaine, Rome, 1976, 915–925.
- (Hijmans 2009, pp. 504–5)
- For a full list of the pontifices of Sol see J. Rupke (ed.), Fasti Sacerdotum (2005), p. 606. Memmius Vitrasius Orfitus lists his priesthoods as pontifex of Vesta, one of the quindecimviri sacris faciundis, and pontifex of Sol, in that order (Corpus Inscriptionum Latinarum vol. 6, 1739–1742). In a list of eight priesthoods, Vettius Agorius Praetextatus puts Pontifex Solis in third place (CIL , 1779).
- The other three were in the Circus Maximus, on the Quirinal, and in Trastevere. (Hijmans 2009, chapter 5)
- Forsythe, Gary (2012). Time in Roman Religion: One Thousand Years of Religious History. Routledge. p. 142-143. ISBN 978-0415522175.
- A comprehensive discussion of all sol-coinage and sol-legends per emperor from Septimius Severus to Constantine can be found in Berrens (2004).
- The medal is illustrated in Jocelyn M.C. Toynbee, Roman Medallions (1944, reprinted 1987) plate xvii, no. 11; the solidus is illustrated in J. Maurice, Numismatique Constantinienne vol. II, p. 236, plate vii, no. 14
- Excellent discussion of this decree by Wallraff 2002, 96–102.
- E. Marlowe, “Framing the sun. The Arch of Constantine and the Roman cityscape”, Art Bulletin 88 (2006) 223–242.
- S. Berrens, Sonnenkult und Kaisertum von den Severern bis zu Constantin I. (193–337 n. Chr.) Stuttgart: Steiner 2004 (Historia-Einzelschriften 185).
- Berrens 2004, precise p. number to follow. The coinage Elagabalus does not use invictus for Roman Sol, nor the Emesan Solar deity Elagabalus.
- Bergmann 1998, 121–123
- S. Hijmans, “Metaphor, Symbol and Reality: the Polysemy of the Imperial Radiate Crown”, in: C.C. Mattusch (ed.), Common ground. Archaeology, art, science, and humanities. Proceedings of the XVIth International Congress of Classical Archaeology, Boston, August 23–26, 2003, Oxford (2006), 440–443; (Hijmans 2009, pp. 80–84, 509–548)
- Bergmann 1998, 116–117; Hijmans 2009, 82–83.
- Hijmans 2009, 509–548. A mosaic floor in the Baths of the Porta Marina at Ostia depicts a radiate victory crown on a table as well as a victorious competitor wearing one.
- Wallraff 2001: 174–177. Hoey (1939: 480) writes: "An inscription of unique interest from the reign of Licinius embodies the official prescription for the annual celebration by his army of a festival of Sol Invictus on December 19". The inscription (Dessau, Inscriptiones Latinae Selectae 8940) actually prescribes an annual offering to Sol on November 18 (die XIV Kal(endis) Decemb(ribus), i.e. on the fourteenth day before the Kalends of December).
- Text at  Parts 6 and 12 respectively.
- (cited in Christianity and Paganism in the Fourth to Eighth Centuries, Ramsay MacMullen. Yale:1997, p. 155)
- Michael Alan Anderson, Symbols of Saints (ProQuest 2008 ISBN 978-0-54956551-2), p. 45
- The Day God Took Flesh
- 1908 Catholic Encyclopedia: Christmas: Natalis Invicti
- Oxford Dictionary of the Christian Church (Oxford University Press 2005 ISBN 978-0-19-280290-3), article "Christmas"
- See discussion in the Talmud (Avraham Yaakov Finkel, Ein Yaakov (Jason Aronson 1999 ISBN 978-1-46162824-8), pp. 240–241), and Aryeh Kaplan's chapter, "The Shofar of Mercy", on the apparent contradiction between that tradition and the Jewish celebration of creation on 1 Tishrei.
- Alexander V. G. Allen, Christian Institutions (Scribner, New York 1897), p. 474
- Frank C. Seen, The People's Work (Fortress Press 2010 ISBN 9781451408010), p.72
- Frank C. Senn, Introduction to Christian Liturgy (Fortress Press 2012 ISBN 978-0-80069885-0), p. 114]
- William J. Colinge, Historical Dictionary of Catholicism (Scarecrow Press 2012 ISBN 978-0-81085755-1), p. 99]
- Joseph F. Kelly, The Origins of Christmas (Liturgical Press 2004 ISBN 978-0-81462984-0), p. 60
- Hippolytus and December 25th as the date of Jesus’ birth
- Christian Worship: Its Origin and Evolution (London: SPCK, 1903), pp 261–265)
- The Origins of the Liturgical Year (New York: Pueblo, 1986), pp. 87–103
- The Flower of Paradise (Oxford University Press 2011 ISBN 978-0-19539971-4), p. 87
- Waiting for the Coming: The Liturgical Meaning of Advent, Christmas, Epiphany (Washington, D.C.: Pastoral Press, 1993), pp. 46–51
- Orthodox Feasts of Jesus Christ & the Virgin Mary (St Vladimir's Seminary Press 1997 ISBN 978-0-88141203-1), p. 20
- Michael Alan Anderson, Symbols of Saints (ProQuest 2008 ISBN 978-0-54956551-2), pp. 45-46
- Hijmans (2009), p. 595
- Hijmans (2009), p. 588
- Adrian Hastings, Alistair Mason, Hugh Pyper (editors), The Oxford Companion to Christian Thought (Oxford University Press 2000 ISBN 978-0-19860024-4), p. 114
- Susan K. Roll, Toward the Origin of Christmas (Kempen 1995 ISBN 90-390-0531-1), p. 107
- Hartmut Miethe, Hilde Heyduck-Huth, Jesus (Taylor & Francis), p. 104
- 26th sermon
- F. van der Meer, Brian Battershaw, G. R. Lamb, Augustine the Bishop: The Life and Work of a Father of the Church (Sheed & Ward 1961), pp. 292-293
- Carl Friedrich Keil, Biblical Commentary on the Old Testament (Eerdmans 1969), vol. 25, p. 468;
- Ephesians 5:14
- Clement of Alexandria, Protreptius 9:84, quoted in David R. Cartlidge, James Keith Elliott, The Art of Christian Legend (Routledge 2001 ISBN 978-0-41523392-7), p. 64
- Weitzmann, Kurt (1979). Age of Spirituality. Metropolitan Museum of Art. p. 522. ISBN 978-0-87099179-0.
- Matilda Webb, The Churches and Catacombs of Early Christian Rome (Sussex Academic Press 2001 ISBN 978-1-90221058-2), p. 18]
- Martin Kemp, The Oxford History of Western Art (Oxford University Press 2000 ISBN 978-0-19860012-1), p. 70
- Hijmans (2009), pp. 567-578
- David R. Cartlidge, James Keith Elliott, The Art of Christian Legend (Routledge 2001 ISBN 978-0-41523392-7), p. 64
- Weitzmann, pp 370, 375
- Berrens, Stephan (2004), Sonnenkult und Kaisertum von den Severern bis zu Constantin I. (193–337 n. Chr.), Geschichte (Franz Steiner Verlag); Historia (Wiesbaden, Germany) (in German), F. Steiner, ISBN 978-3-515-08575-5
- Hijmans, S (2003), "Sol Invictus, the Winter Solstice, and the Origins of Christmas", Mouseion Calgary 3.3: 377–398, ISSN 1496-9343, OCLC 202535001
- Hijmans, Steven E (2009), Sol : the sun in the art and religions of Rome (Thesis/dissertation), ISBN 90-367-3931-4
- Matern, Petra (2002), Helios und Sol : Kulte und Ikonographie des griechischen und römischen Sonnengottes (in German), Ege Yayınları, ISBN 978-975-8070-53-4
- Weitzmann, Kurt, ed., Age of spirituality : late antique and early Christian art, third to seventh century, 1979, Metropolitan Museum of Art, New York; fully online from The Metropolitan Museum of Art
In Popular Culture
- The thriller 'Sol Invictus' by J. Paul Roe makes numerous references to Saturnalia and the evolution of ancient holidays.
|Wikimedia Commons has media related to Sol Invictus.|
- Encyclopædia Britannica Online: Sol
- Probus and Sol, includes images of coins
- Roman-Emperors: Aurelian
- Gibbon's Decline and Fall: Triumph of Aurelian
- Gibbon's references for Aurelian's Temple of Sol Invictus
- Clement A. Miles, Christmas in Ritual and Tradition, Christian and Pagan (1912): December 25 and the Natalis Invicti
- Catholic Encyclopedia (1908): Christmas
- Ancient sources