centre of gravity:

(aka centre of mass) In physics, the centre of mass of a distribution of mass in space is the unique point where the weighted relative position of the distributed mass sums to zero, or the point where if a force is applied it moves in the direction of the force without rotating.  Spengler is making an analogy with “centre of time”, both terms being scientific (cause/effect school); he implies that a historian (using cause/effect or cultural bound ) will have a fixed perspective, which is "centre of time".

 

sub specie aeternitatis:

(Latin for "under the aspect of eternity") Spinoza on wards, an honorific expression describing what is universally and eternally true, without any reference to or dependence upon the temporal portions of reality; roughly means "from the perspective of the eternal", often used to describe an objective point of view.

 

historical heads Nurnberg cathedral: * see Endnote 2

Spengler mistakenly refers to a "Cathedral" at Nurnberg which might refer to a number of Gothic churches: the Frauenkirche, St Lorenz  & several others.  The church with the best preserved sculpture is St Lorenz.  A medieval church  dedicated to Saint Lawrence, the nave was completed 1400.  In 1439, work began on the choir in the form of a hall church in the late German Sondergotik Gothic architecture style, completed 1477.  The choir holds the carving of the Angelic Salutation by Veit Stoss, and the monumental tabernacle by Adam Kraft. The latter includes a prominent figure of the sculptor himself.

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heads (portraits of) Durer: * see Endnote  3

German artist Albrecht Dürer (1471-1528) was the greatest artist of the Northern Renaissance. As a draughtsman and painter, he rivaled his elder contemporary Leonardo da Vinci;  his woodcuts and engravings of mythical and allegorical scenes made him famous across Europe.

 

heads (portraits of) Rembrandt: * and see Endnote  4

Rembrandt was a popular portrait painter; he began this work in 1631 in Amsterdam (new business capital of the Netherlands); he enjoyed great initial success & 30 years later was still fulfilling major commissions for portraits.  

 

Hellenistic sculpture of Sophocles: * and see Endnote 5

"the famous Sophocles statue": in a somewhat vague reference Spengler is possibly referring to a Roman copy, at Musei Capitolin Rome.