A short blog to mark a long day.
A curved line is a beautiful thing. Especially when it is both convex and concave, inward and outward, especially when it shows something as magical as the journey of the Sun across the Earth. This path is called the ecliptic. Today is the Summer Solstice*, the day of the longest amount of sunlight for those in the Northern Hemisphere. We blogged, here, about the Spring equinox, using a beautiful Dutch double hemisphere map of the World from an atlas by Claes Janszoon Visscher. In that blog we highlighted the straight line of the Equator, crossed by the Sun twice in a year to give us the Spring and Autumn equinoxes, days of equal day and night.
Planisphærium Terrestre sive Terrarum Orbis…from Atlas Minor by Claes Janszoon Visscher, c1705. Map Res. 85
Here we follow the curved line, showing the Sun’s journey through the Heavens, the ecliptic. When the Sun is at its most northern point, today, it reaches the Tropic of Cancer, and marks our Summer Solstice. It’s both obvious and amazing that it’s not the Sun though making this curved journey. The Sun stays where it is within the Solar System, even though, along with all the other objects in the Solar System we’re travelling at 450,000 miles per hour around the Milky Way. It’s the Earth that moves, tilting on it’s axis throughout the year and it’s this variation of tilt towards the Sun that produces this curved path in relation to the Earth, and from this our seasons.
Here’s a diagram from a 1909 edition of ‘Bacon’s popular atlas of the World’, showing the workings of the Solar System. ‘Astronomical diagrams’ shows not only the seasons but also a diagram showing the difference in the spread of the Sun’s rays at the two solstices, a concentrated 15° on June 21st (or 20th in this year) and a wider and hence weaker 28° a the winter solstice. Surely this though depends on whereabouts on the Earth you are?
Bacon’s popular atlas of the World, 1909. G1.B1.52
Bacon’s atlas uses the Patent thumb-index, which the publisher claims has been ‘invented specially for this work’. Along the right-hand edge of the atlas the margin gradually gets cut away as you journey through the atlas. This allows you, with a sliding motion up with your thumb, to open up the atlas at any of the general maps you want, Africa for instance, and from there see a list of maps of Africa and the countries therein, followed by the country maps of that area. Bacon promises, ‘This important improvement, it will be seen, has thus been effected without disfiguring the edge of the book’. Probably helps here to have reasonable finger-nails.
*This year’s solstice is the earliest since 1796. It’s because 2024 is a leap year, which means the solstice is 18 hours earlier than in a non-leap year. That it’s as early as it is is due to some complex maths based on how the Georgian calendar tries to fit in leap years over the course of centuries.