Current Sky: Summer Solstice On Nantucket
Regina Jorgenson •
The month of June brings one of the most anticipated - and one of my favorite - days of the year, the solstice. In the northern hemisphere, this day is also known as the summer solstice, the first day of summer, and the longest day of year. This year the solstice takes place on Wednesday, June 21 at 10:58am Eastern.
So, what does the solstice mean, and why should we care? To start, recall that the Earth’s axis is ‘tipped’ at 23.5 degrees with respect to the plane of our solar system. Over the course of a year, as the Earth orbits around the Sun, the axial tilt remains pointed in the same direction, towards the star Polaris, also known as the North Star. The solstices mark the two positions in the Earth’s orbit around the Sun when the axial tilt is pointed most directly towards or away from the Sun.
On the summer solstice in the northern hemisphere, the North Pole of the Earth’s axis is tipped most directly towards the Sun. As a result, on this day the northern hemisphere receives the most direct sunlight for the longest amount of time. This extended time of more direct sunlight heats up the northern hemisphere causing the warmer weather of the summer season. In practice, there’s about a month’s-long delay to the warming, as the thermal inertia of Earth’s surface and oceans means that they take a little while to heat up – which also explains why the months of August and September are typically warmer than May.
The word solstice is derived from the Latin words ‘sol’ and ‘sistere’ which mean, respectively, ‘Sun’ and ‘to stand still.’ The ‘standing still’ refers to the apparent motion of the Sun’s path in our sky over the course of the year. In the northern hemisphere, the Sun’s daily east-west path gets higher and higher in the sky as we progress towards the summer solstice. It then peaks to its highest path on the summer solstice, apparently standing still or stopping in its progression, and then progressively decreases again until winter solstice, when it reaches its lowest point.
What that means for us Earth-bound observers is that on summer solstice the Sun appears the highest in the sky that it will ever reach at noon on the day of the solstice. Correspondingly, shadows are the shortest that they’ll ever be on the summer solstice. Afterwards, the path of the Sun reverses course, and each day the Sun’s highest point is slightly lower in the sky, creating a corresponding decrease in the total amount of direct sunlight received in the northern hemisphere.
There exists a slight subtlety to the definition of ‘noon,’ thanks to our human-made time zones. Here we are speaking of ‘solar noon,’ which is when the Sun appears to cross the local meridian, or the imaginary line running from north to south and passing through both the North Star and the zenith. For reference, on Nantucket, solar noon will occur at 12:42pm Eastern on June 21 of this year.
Here on Nantucket, we can easily find our local meridian thanks to William Mitchell, the father of Nantucket’s famous daughter, Maria Mitchell, the first American woman astronomer. Among his many professions including teacher, astronomer, and cashier at the Pacific National Bank, William Mitchell was also a land surveyor. He understood the importance of knowing the cardinal directions, as well as the problems inherent with using a compass, which points towards the Earth’s magnetic North Pole. Over time, the Earth’s magnetic North Pole moves, at a rate of approximately 30 miles per year. Therefore, knowing the direction of true geographic north is key to determining the necessary correction to the magnetic north direction indicated by a compass.
In 1840 William Mitchell erected the first meridian stones on Nantucket to indicate the true geographic north-south line. The northern-most of these stones, which are still standing today, is located next to the Pacific National Bank at the top of Main Street. The southern-most stone is located approximately 300 feet to the south, up Fair Street and near the old Quaker Meeting House. In 2006, the former Maria Mitchell Association Director of Astronomy Dr. Vladimir Strelnitski published an article[1] in American Surveyor magazine, summarizing a multi-year investigation that he undertook, with the help of collaborators, to solve several mysteries surrounding the ‘Mitchell Meridian Stones,’ and to debunk several of the commonly held misconceptions surrounding their purpose.
So, why does the Earth’s axis tilt at 23.5 degrees with respect to its orbit around the Sun? While we don’t know for certain, the most widely accepted theory is that early in the life of our solar system the young and forming planet Earth was hit by another large object approximately the size of the planet Mars. This object, called Theia, collided with the Earth, knocking over the axis of Earth’s rotation and causing a large amount of debris to be ejected. Eventually, the ejected debris coalesced to form our Moon.
Whether due to the impact of the hypothetical proto-planet Theia or not, we have the Earth’s axial tilt to thank for the changing seasons. Without this axial tilt, Earth would not experience seasonal temperature and weather changes, and summer on Nantucket would not exist! So, as we gear up to celebrate this summer’s solstice on June 21, be sure to swing by the Nantucket Meridian Stones to check out a bit of Nantucket history. If the Sun is out, take special note of the stones’ shadows, which, at local solar noon (12:42pm) will be the shortest you’ll ever see them.