Summer is now truly upon us with the longest day of the year in the Northern Hemisphere occurring on June 21. Since the Winter Solstice in December, the Sun has been rising higher in the sky each day, but on June 21 this behaviour stops and the sun is said to “stand still”. The word ‘solstice’ is derived from the Latin solstitium, from ‘sol’ (sun) and ‘stitium’ (to stop).
One somewhat overlooked consequence of this motion is that the sun never sets very far below the horizon at this time of the year as seen from Ireland, and the night-time skies never fully darken. This is generally not good news for skywatchers as the glow from the sun washes out fainter objects.
The sun is not the only object this month that seems to change direction. The giant planet Jupiter has been moving slowly east against the background stars since roughly February 6, but on June 10 that motion ceases and Jupiter appears to reverse its direction and move westward relative to the background stars. This subtle change of direction might not seem important to the casual observer, but in the words of the science writer Isaac Azimov: “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ but ‘That’s funny...” And this “funny” change in Jupiter’s motion turned out to have huge consequences for our understanding of how our solar system, and ultimately our universe, works.
For more than 2000 years, the Greek philosopher Ptolemy’s model of the solar system held sway. In it, the Earth was at the centre of the entire universe and everything orbited the earth in perfect circles. The problem is that this model does not easily account for the change in direction of Jupiter (known as retrograde motion). For it to work, Ptolemy introduced the idea that as Jupiter went around the Earth in a perfect circular orbit, it also went around another smaller circle attached to that orbit called an epicycle. While this addition to his model worked initially — for quite a long time, actually! — it began to break down as better observations by naked-eye astronomers required more and more epicycles to be added to explain the movement of Jupiter in ever greater detail.
And when Nicolas Copernicus suggested that the sun is at the centre of the universe and Earth and all the planets orbit around it, the need for complicated and arbitrary epicycles disappeared in one fell swoop. The story is a nice example of Occam’s Razor, a hypothesis that says if two or more theories explain a phenomenon equally well, the simplest is likely to be the correct one. Look for Jupiter throughout June in the southwestern evening sky. It is the brightest object in the night-time sky after the Moon.
From June 26-August 25, the largest ever gathering of space scientists from around the world will take place in Cork, hosted by Cork Institute of Technology. Operated by the International Space University, the nine week programme of events includes mroe than 50 public events, many of which are being coordinated by the CIT Blackrock Castle Observatory under a banner of a ‘Summer of Space’.
More Irish companies are seeking new markets for their products and processes, and the Space Studies Programme will make Cork the place to be if you want to find out more from some of the world’s greatest space experts. There is something for everyone, across all ages.
For details on events, keep an eye on www.ssp17.ie and www.bco.ie .
Did you know?
A day on Venus is longer than a year on Venus!
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