Sir Joshua Reynolds, although president of the Royal Academy and the foremost portrait painter of the 18th century, was sometimes unwise in his choices of materials and techniques. If the faces of some of his figures nowadays seem deathly pale, it is largely because the carmine pigment he used has faded. This emerging flaw caused the poet William Blake to observe with uncharacteristic whimsicality:
When Sir Joshua Reynolds died,
All Nature was degraded;
The King dropped a tear in the Queen's ear,
And all his pictures faded.
Reynolds was also ridiculed by more avant garde contemporaries like Millais and Dante Gabriel Rossetti for the traditional lack of detail in the backgrounds of his paintings. These stalwarts of the so-called PreRaphaelite Brotherhood believed every detail of a scene to have significance and painted everything into sharp focus; Reynolds, by contrast and following the practice of his time, would provide detail only in the foreground and simplify the background.
The Pre-Raphaelites coined the term "sloshy" to describe this allegedly passe technique, and the sobriquet "Sir Sloshua" to identify its most illustrious proponent.
This neglect of background detail is nowhere more evident than in the well-known painting by Sir Joshua called Cupid Unfastens the Belt of Venus. The nondescript backdrop to the scene focuses our attention on the naughty little cherub, who sits beside the reclining Roman goddess, deshabille, and who is doing exactly what the title says he does. To meteorologists, however, the "belt of Venus" is something altogether more mundane and less risque.
On an evening when the sky is cloudless and the weather hazy, it is sometimes possible to see, just after sunset, the shadow of the Earth projected against the sky over the eastern horizon. It appears as a dark, bluish-grey horizontal band rising slowly upwards shortly after the sun in the west has disappeared. The more hazy the atmosphere, the better it acts as a screen on to which this shadow is projected.
High up, of course, the sky will still be blue, its normal colour resulting from the scattered sunlight which the atmosphere continues to intercept throughout the twilight.
But in between this area of blue and the dark eastern shadow down below is a narrow band - a horizontal fringe to the shadow, as it were - which is a very definite shade of reddish brown. It occurs because in this narrow zone the light being scattered in the direction of the observer by the atmosphere at that point is the red light of the setting sun.
This reddish boundary between the dark and blue is called "the belt of Venus".