At this time of year, those of us that experience four seasons are quite ready to move on to the next one. We’re hoping for any signs we can find to reassure ourselves that Spring is on the way, no matter what the Groundhog says.
That’s especially true this year, as the nation tries to emerge, and not only weatherwise, from a long, cold Winter. Spring promises the beginnings of a return to normal. As it begins to warm up a bit, we’ll be able to sit outside with our friends again and look forward to happier and healthier times to come.
Yes, it’s still cold here in New England, but this past week, for the first time since Fall, the Sun set at 5:00. We’re on our way out of the darkness.
Nature may not have presaged the arrival of Spring with warmth just yet, but it has another clue for us if we know where to look: the rise of our great friend, the Big Dipper.
The Big Dipper (an asterism within Ursa Major). Alcor and Mizar appear very close together from our vantage point.
The Big Dipper circles slowly around the North Star, not only as the night goes on, but also as the seasons change, appearing highest in the sky in the springtime. Below we see what the sky looks like at 10 P.M. at various times of year when we face north, standing at about 40° north latitude. The farther south you go, the lower in the sky the Big Dipper will be, disappearing below the horizon altogether in the Fall in places like Miami.
But now, in February, if you look to the northeast, the Big Dipper is rising, standing on its handle.
FEBRUARY 5
February 5, 10 P.M., Boston. Pink: Big Dipper; Yellow: Polaris.
MAY 5
May 5, 10 P.M., Boston. Pink: Big Dipper; Yellow: Polaris.
AUGUST 5
August 5, 10 P.M., Boston. Pink: Big Dipper; Yellow: Polaris.
NOVEMBER 5
November 5, 10 P.M., Boston. Pink: Big Dipper; Yellow: Polaris.
If you can stay up until 2 A.M., the February picture will look a lot like the May picture, with the Big Dipper high in the sky.
We can use the Big Dipper, of course, to locate the North Star (Polaris). If you imagine a line through the two “dipper” stars opposite the handle, Merak to Dubhe, and keep following it to the first bright star, that’s Polaris.
The flag of Alaska
No matter where you are, facing the North Star means facing due north. These things were well-known to slaves in the 1800’s in the South, who sometimes referred to the Big Dipper as the Drinking Gourd. Hollowed-out and dried gourds were used by Native Americans (actually still are!), and also for millennia in Africa, as utensils, containers, instruments, and other things. The “drinking” variety looks a lot like the Big Dipper:
A “drinking gourd”
There is an old song called “Follow The Drinking Gourd” that slaves may have used as a sort of coded map to freedom. There are many versions of it, and it didn’t appear in print until 1928, so it’s difficult to know whether it was actually utilized that way. But there is no question that slaves were well aware of the relationship of the Drinking Gourd to Polaris, and they absolutely used that knowledge to escape to the North without a map, especially because they usually had to travel by night, and the journey could take many months.
From “Scenes in the Life of Harriet Tubman” by Sarah Hopkins Bradford (1869):
“Harriet Tubman: The Road to Freedom” by Catherine Clinton (2004):
Frederick Douglass started his own anti-slavery newspaper in 1847, and he called it…
RIGHT IS OF NO SEX — TRUTH IS OF NO COLOR — GOD IS THE FATHER OF US ALL, AND ALL WE ARE BRETHREN.
… and in the first issue he explained why this name was chosen:
NORTH STAR.
Of all the stars in this "brave old, overhanging sky," the NORTH STAR is our choice. To thousands now free in the British dominions it has been the STAR OF FREEDOM. To millions, now in our boasted land of liberty, it is the STAR OF HOPE. Dark clouds may conceal, but cannot destroy it. Tempests may toss the sea — earthquakes convulse the globe — and storm-bolts shake the sky — it stands as firm as Heaven. Within its meek and twinkling rays, are Faith, Hope and Freedom — cherishing the one, indulging the other, and endeavoring to gain the last for our slavery smitten countrymen.
We have ventured to call our humble sheet by our favorite Star. We have been requested to change it, but as yet see no good reason for doing so. The Morning Star was suggested; the Evening Star has been named, but the one is too early, and the other too late. The Midnight Star is our election. We are over-shadowed by gloomy clouds, and on a dark and perilous sea. We need the Polar Light to guide us into port.
Artist’s rendering based on Hubble Space telescope image of Polaris, a triple star system consisting of the yellow supergiant Polaris Aa and two white (spectral type F) main-sequence stars, Polaris Ab and Polaris B. Polaris Aa and Ab are in close orbit with each other, and Polaris B orbits the pair.
The North Star can tell us a lot more than simply which way north is. It’s directly above the Earth’s spin axis, so if you’re at the North Pole, it’s directly overhead, and if you’re at the Equator, it's right on the horizon. But if you’re in between those two, you can measure the angle from the horizon to the North Star, and voilà! That is your latitude. You’d use a sextant to do this in person, but let’s look at another map with gridlines every 10 degrees and figure out Boston’s latitude this way:
The sky tonight in Boston
The grid squares are 10 degrees by 10 degrees, so we can just count up the number of squares we need to traverse from the horizon to reach Polaris. We count four whole squares, and then about a quarter of a square, so we estimate that Boston is at 42.5° north latitude. The real answer is 42.36°. Not bad a-tall.
Polaris hasn’t always been the North Star, and it won’t be anymore after about the year 3000. That’s because the spin axis of the Earth slowly precesses (over a period of about 26,000 years) like a spinning top:
The tilt of the Earth’s spin axis changes slightly over time, too:
Knowing this, we can project past and future North Stars, like Deneb around the year 10,000 and Vega in about 14,000. (Will people use commas in years after 9999?) These stars won’t be as nearly dead-center as Polaris is now, but they’re much brighter and easier to spot.
Position directly above Earth’s spin axis relative to stars
You might recognize Vega and Deneb from Summer Triangle fame...
The “Summer Triangle”
...but that’s a story for another season.
