On January 1st, much of the world will be celebrating the new year, but this Friday might be a more appropriate day to celebrate (at least from an astronomical viewpoint).
This Friday (22 December 2018) marks the Winter Solstice, the date on which the calendar changes seasons from Fall to Winter here in Mid-Michigan (and the rest of the Northern Hemisphere). Winter will officially begin at 5:23 PM EST.
The word
solstice comes from two Latin root words
sol (sun) and
sistere (to stand still). On the solstice, the path of the sun appears to stand still!
Assume that you were to awaken every morning before dawn and mark the location that the sun rises on the eastern horizon. You would notice that the sun does not in fact arise from the same place every day. Instead it wanders north and south along the horizon, moving north during the Winter and Spring and back south again during the Summer and Fall. On the Summer Solstice (June 21st) and Winter Solstice (December 21st), the sun ceases to move further along the horizon, appearing to "stand still" before reversing its course the next day.
You have probably already noticed in addition to the location of the rising sun, the length of day and night vary throughout the year. As the sun progresses south, the day shortens and the night lengthens. The Winter Solstice marks the shortest day (and longest night) of the year in the Northern Hemisphere. Here in Mid-Michigan we will mark the date with approximately 8 hours 57 minutes of daylight. As the sunrise moves back north along the horizon our days will slowly get longer until on the Summer Solstice we will experience nearly 15 hours 25 minutes of daylight.
Why does the sun move and the length of day vary?
It all has to do with a tilt in the Earth's axis. The Earth rotates on its axis approximately once every 24 hours - giving us the length of our day. It also revolves around the sun approximately once every 365 days - the length of our year. If the Earth's axis was perpendicular the plane on which it revolves around the sun, our days would be the same length throughout the year (approximately 12 hours), but the axis is not perpendicular to this plane. Instead the Earth's axis tilts 23.5 degrees from the vertical.
The
points on the globe that the axis revolves around are referred to as the
North and South Poles. The axis is always pointed toward the same
location in the sky. The North Pole points toward the "North Star" -
Polaris. As the earth revolves around the sun, sometimes the North Pole is closer
to the sun, sometimes the South Pole is closer to the sun. When the
North Pole is at its closest, we experience Summer in Mid-Michigan and
the Southern Hemisphere experiences Winter. When the North Pole is at
its furthest, we experience Winter and the Southern Hemisphere
experiences Summer.
On two days of the year, the Earth's axis is perpendicular to the plane of its revolution. On those two days we
do experience equal periods of day and night because the sun rises due east and sets due west. During the rest of the year our hours of daylight vary depending on where the axis is pointed. During our summer months, the tilt in the axis means that the northern hemisphere is closer to the sun and receives more hours of sunlight. During the winter months, the reverse is true. On the Winter Solstice, the North Pole is tilted at its furthest angle from the sun resulting in the shortest day (and longest night) of the year for those of us in the Northern Hemisphere.
Why did I say that the Winter Solstice would be a better day to celebrate the new year?
Tracking the position of the rising sun was among the first astronomical observations. Many ancient monuments seem to have been constructed as solar observatories, aligned with the locations of sunrise and sunset on the longest and shortest day of the year. To early agricultural societies especially, tracking the seasons accurately could mean the difference between a bountiful harvest and failure of a year's crops. The day that the sun "stood still" in its southward journey and then began to return north, bringing more hours of daylight and eventually warmer weather seems such an obvious point to mark the beginning of a new year!
Join me on a hike to celebrate the Solstice!
To celebrate the Winter Solstice, I am leading a nighttime hike at the Chippewa Watershed Conservancy's
Sylvan Solace Preserve. This hike will begin at 7:00PM. Sylvan Solace Preserve is located on Pickard Road, between Gilmore Road and Littlefield Road, approximately 8 miles west of downtown Mt. Pleasant.
We are doing a nighttime hike because another celestial event lines up with the solstice this year - a full moon. (Technically the full moon will be on December 22nd, but it will be more than 97% illuminated on the night of the 21st.) We are expecting light snowfall early in the day - this should brighten up the woods. Hopefully the moon will peek through the expected clouds and illuminate our hike. The trails at Sylvan Solace are flat and well-defined making this a great first night hiking experience! I hope to do the hike without any artificial lights. Wear warm clothes and bring a headlamp or flashlight just to be safe.
