Sunset at equinox

Today is 22 September 2020, which means it is autumnal equinox in Norway. For those who note the day’s passing on the calendar, equinox might mean to you that night and day are both 12 hours long. Since Marie and I moved to Norway, equinoxes have a new meaning to me. I’m using today’s equinox as a celebration of the shift of season and inspiration for a scientific creation.

My three siblings and I live on three different continents, east and west of the Prime Meridian, north and south of the Equator, with and without summer time (“daylight saving time” in the USA). Family chatter easily turns to local weather, season, and time of day. As we in Norway head to colder and shorter days, my sister in Lesotho finds days getting warmer and longer.

I spend 10-20 hours each week outside and on the move. It changes my brain. I mark time by the sun. I feel the passing of the day rather than measuring it with a clock. I see new seasons by color and weather rather than by looking at a calendar. In our high latitudes, the day is 5 hours long at winter solstice and 20 hours long at summer solstice. Equinox is halfway between. Moreover, equinox is the day when the whole world experiences a 12-hour day. My whole family gets an equal share.

Imagine yourself as a toy figurine standing at the North Pole of a globe, far away is a bare light bulb shining. Today, at equinox, our light-bulb sun traces a path over the equator. If your globe is on a stand mounted at an angle, you’ll find the only way to make this work is to point the tilt perpendicular to the direction of the sun making a funny “L”. This arrangement is the astronomical setup for equinox on Earth.

You see the light rotate around you as globe spins – at the poles on equinox the sun traverses the horizon all day. Now walk your figurine self down to where I am in Norway. The Earth itself blocks the light bulb for half the rotation – there are 12 hours of daytime and 12 hours of nighttime. Walk to the equator and the situation is the same.

Even though the length of day in Norway and at the equator on my fantasy globe are the same, something special happens at sunrise and sunset near the poles. Because the sun never gets very high in the sky, not even at midday, it spends a longer fraction of the day near the horizon. For my travelling soul, on this real planet, that means sunrise and sunset last a long time.

I wonder, how long is a sunset? During our tours under the midnight sun in Kjerringøy, we noticed that the sun glided across the northern horizon for hours and hours. Sunset lasted all night. On our visits to the tropics, the sun sinks like a stone. Marie and I kicked around the idea for a few days as I tried to figure out how to plot it as data on a graph.

I’ve chosen to plot the length of sunset as a bar graph displayed between two hemispheres of the globe. The locations are all places where I have spent time paying attention to the sunset. You can see that, even though equinox gives 12-hour days across the globe, there is a spread in how long it takes the sun to sink below the horizon. Sunset today at our new home in Folldal, Norway takes 22 minutes but 13 minutes for my mom in Amarillo, Texas, USA.

I chose my own definition of sunset. This representation assumes that sunset starts when the sun is within 2° of the horizon. The sun itself subtends about 0.5­° of the sky, so my definition uses about 4 suns. I’ve made an arbitrary choice, but it’s sensible and representative for my purposes. I pulled data from www.timeanddate.com, a website I’ve been using for years.