Realism

As the 14th century progressed, the people of the Northern Countries shifted away from the scholastic philosophy of religion to the autonomy of human experience.
Jan van Eyck's paintings are highly glazed with a divine reality, and the environments were carefully constructed (although none of his interiors are of any one place in particular, which is unique to Gothic art.) According to Craig Harbison in his "Realism and Symbolism in Early Flemish Painting" (The Art Bulletin vol. 66 no. 4), van Eyck was not only interested in recording, but rather he was more involved with interpreting data and turning it into supernatural truth.

Harbison also points out that not one of van Eyck's paintings contain an earthly event or place. All of his scenes take place around divine happenings, and all earthly objects are infused with hidden symbolism. His aim was to "remind one, even lead one, to ultimate or universal truth." It can be seen that the detailed and exquisite renderings of biblical themes and narratives would bring one closer to the actual "event", using only the characters as groundwork for the believer's reality.

Portraiture

Van Eyck presented himself as an innovator, or trend setter, of his times. Along with mastering the use (not inventing, like originally thought) of oil paints in his workshop, going on secret missions for Dukes, being a Realist (vs. a Nominalist),  he was also the only one of his time to paint secular portraits. 

These occurrences, the depiction and subsequent "immortalization" of the subject at hand without any ties or references to the church of Christianity, were most likely due to van Eycks' ties to the court. His popularity, at that, can also be attributed to his being in the service of Philip the Good.  Eyck's stylization was structured around the tastes and likings of the people he was commissioned by, which takes away from individual expression.

Earliest known painting of the moon

According to a scientist in Seattle named Scott Montgomery, Jan van Eyck's painting of the moon replaces Leonardo da Vinci's sketch by 100 years. Van Eyck had a penchant for depicting the moon in his paintings, mostly due to his philosophical stance as a Realist. 

If you look closely to the right, you will see the moon with recognizable surfaces.  And although the texts will give credit to Leonardo da Vinci as the first to depict the moon naturalistically, Montgomery says that they should be corrected, as van Eyck painted the moon in four different works (the above being the most popular.) (source: http://news.bbc.co.uk/1/hi/sci/tech/399918.stm)

As a realist, van Eyck wanted to show his world as it really was, only completely saturated with a divine presence. Thus, we see in his works a perfect concentration to detail, and a pictorial quality that is full of symbolism, both hidden and available.

Beginnings

Jan van Eyck was traditionally trained with his brother, Hubert, in the Ghent tradition. Although his date of birth is not known, he is thought to originate from the town Maaseik in Belgium (then called Limburg).  

From 1422 -24 van Eyck was first a court painter for John of Bavaria, Count of Holland at the Hague. Following the count’s death, he went into the service of Philip the Good, Duke of Burgundy. Philip used van Eyck as a secret agent of sorts; he sent him on a mission to retrieve the Isabella Infanta’s hand in marriage in Portugal. There he stayed from October 1428, until December 1429, and although he painted during his stay, he only knowingly completed two portraits of the Infanta, and painted the original Fountain of Life. Van Eyck later used her dress in the famous Ghent Altarpiece on the Eritrean sibyl.