February is the month we traditionally acknowledge Americans of African origin, and the plight of their ancestors
held as slaves during the early years of our country. During the years leading up to and around the American Civil War, slaves very often
made their way north to freedom by taking advantage of the Underground Railroad,
a system of people and locations that were safe havens or safe routes for the escaping slaves to follow.
Much of the travel was done at night and as was common in those days, people used their
knowledge of the stars to help determine compass directions. Primarily used were the stars of the two dipper-shaped patterns known as the
Big Dipper and the Little Dipper. These easily recognizable star patterns, or asterisms, are parts of the constellations Ursa Major,
the Great Bear, and Ursa Minor, the Little Bear. The Big Dipper asterism was also known to many as the 'Drinking Gourd' due to its
resemblance to a common gourd that, when dried out, was used as a drinking vessel.'
Seasonal Motion vs. Circumpolar Motion Stars that we see at night may be classified into two groups based on their visibility. Stars that rise in the east and
set in the west and are highest over the southern horizon (for the Northern Hemisphere) may be referred to as seasonal stars. As the Earth
revolves around the Sun, these stars and their associated constellation patterns rise earlier each night by approximately four minutes, or
by about two hours each month. What we will see as winter stars in the evening this month, for example, will be seen farther toward the west
at the same time next month, and even further to the west the next month, and so on until those stars are no longer above the horizon during the night hours.
(Looking south on the 15th of January, February, March, and April at 11p.m.) To see these westward seasonal changes go outside around 8:00 p.m., midmonth (February), and observe where the stars of Orion are located.
Repeat this observation next month at the same time and you will see that Orion is farther west.
The other group of stars classified by visibility above the horizon are referred to as circumpolar stars. These stars and
their associated constellation patterns never rise or set relative to the horizon, but are visible all night long. They follow a circular pattern around
a point in the sky known as the celestial pole. In the Northern Hemisphere the north celestial pole is marked by the star we call the North Star, or Polaris.
It is the end star in the handle of the Little Dipper, and is very close to being directly over the Earth's North Pole. As the Earth rotates about
its axis of rotation, the circumpolar stars appear to follow a circular motion around Polaris, neither rising nor setting.
Circumpolar stars may also be thought of as seasonal stars in that they will have a certain position over the Northern
Hemisphere each season. However, since their apparent motion carries them around the pole star throughout the night it is more practical to
consider these changing seasonal positions by using a particular time as a reference.
(This animated graphic shows a north view on the 15th of each month - starting with January) With that in mind, during the evening hours, after sunset,
for example, the seasonal position of the stars making up the Big Dipper pattern may be described with reference to hours on a clock. During the winter
months the Big Dipper is located at about the three o'clock position; during spring it is high overhead, near the twelve o'clock position; during
the summer months the Big Dipper is near the nine o'clock position; and during the autumn the Big Dipper is low over the horizon, near the
six o'clock position
Using an online rotating Big Dipper Star Clock pattern you can explore the changing position of
these stars due to the rotation and revolution of the Earth may be used as a sky clock for approximating the local time due to the regular
rate that the Earth rotates.
Use this online 'Big Dipper' clock to determine the position of the 'Drinking Gourd'.
Print out your own star clock.
It is the end star in the handle of the Little Dipper, and is very close to being directly over the Earth's North Pole. As the Earth rotates about
its axis of rotation, the circumpolar stars appear to follow a circular motion around Polaris, neither rising nor setting.
