BASIC CELESTIAL PHENOMENA
Putting the Universe in Motion
Daily (Diurnal) Motion
If we were to spend several hours under a starry sky, we might begin to see more things
that our brief running-to-the-car glimpses give us. The sun, moon, planets, and stars
would all appear to move slowly westward across the sky. If we observed even longer we
might figure out that the entire cycle of this drift repeats every 24 hours. Most celestial
objects rise roughly in the east, ascend in the eastern sky until they reach their maximum
height, and then descend in the western sky until they set roughly in the west, in what is
called their daily (diurnal) motion.
Closer observation would reveal that east-to-west isn’t quite correct for all objects. There
is a point low in the northern sky around which objects seem to circle. In fact, there is a
small, insignificant star near the center of this circle. This North Star appears virtually
motionless relative to the other stars circling it. Circumpolar objects are stars or
constellations that never rise or set as observed by someone in one of the two
hemispheres—northern or southern (we’ll concentrate on the northern hemisphere from
now own, since that’s where we are). A circumpolar celestial object never dips below our
northern horizon. It simply circles the North Star. Two of the most famous and easily
recognizable circumpolar constellations are Ursa Minor (Little Dipper), which includes
Polaris (the North Star); and Ursa Major, which contains the Big Dipper.
Navigation by Stars
From a given location on Earth, a star rising above a
particular reference point on the horizon on a given
day will appear to rise at that same location every
day. Sailors learned to guide their ships according to
the rising and setting locations of fixed stars. This
allowed them to determine their own position in
latitude relative to their home or harbor. For example,
the Pilgrims who landed in Plymouth Rock expected
a relatively mild winter, comparable to that of
England, for during their ocean journey they had
maintained their ship’s direction on the same latitude. However, due to unanticipated
effects of ocean currents, the New England winter is much harsher than that in the same
latitude in England. Arabian nomads relied on the stars to navigate across trackless desert
sands. The Quran affirms (6:95): “It is God who has appointed for you the stars, that by
them you might be guided in the shadows of land and sea.”
“Lift your eyes and look to the
heavens: Who created all
these? He who brings out the
starry host one by one, and
calls them each by name.
Because of his great power
and mighty strength, not one
of them is missing.”
Isaiah 40:25
Big Bear (Ursa Major) and the Big Dipper… URSA MAJOR (Big Bear) is among the
largest of the 88 officially recognized constellations. Several stars form a smaller group
within this constellation, variously called the “Big Dipper” (America), “The Plough”
(Britain), or many other names. Any such recognized subset or “smaller piece” of a
constellation is known as an asterism.
The Big Dipper is one of the most easily recognizable groups of stars in the sky and thus,
despite its status as a mere asterism, is widely considered a constellation in its own right.
The “handle” of the Big Dipper represents the “tail” of the bear even though bears don’t
have long tails. (A Greek myth explains that Zeus stretched out the bear’s tail and placed
it in the sky. If you’re going to believe in a bear in the sky, you may as well believe it had
a long tail.)
For people living north of Shawnee’s latitude of 35° the Big Dipper is circumpolar (never
setting below the horizon) and therefore visible in northern skies year-round. For people
living at Shawnee’s latitude of 35° there will be times when part of the Big Dipper will
be obscured by the horizon, or by the haze of city lights on the horizon, or by local
horizon features such as trees. You can learn, as did ancient sailors or western cowhands
on the night watch, to tell the time of the night by the position of the Big Dipper. Due to
the daily rotation of the Earth, the dipper appears to rotate around the North Star (Polaris)
every twenty-four hours.
Describing the location of a celestial object…
1. By using the Horizon Circle
Imagine yourself in a large, flat field. If you point toward any spot on the horizon then
pivot completely around, your hand will trace a 360o circle along the horizon and end up
at the starting point. If we assign the value 0o to the point on the horizon that is due
North, then that puts East at 90o, South at 180o and West at 270o. We can also assign
degree numbers to any points in between the others. These horizon values are called the
azimuth. If a star appears to be directly in line vertically with a certain point on the
azimuth, such as 45o, then it has an azimuth of 45o.
We can use a similar scale to define the vertical
position of an object. Point at an object in the sky.
The angle your arm creates relative to the flat
ground is known as the altitude. An object may
have an altitude between 0o (right on the horizon)
and 90o (straight overhead, or zenith)
2. By using the Celestial Equator
Imagine the earth having a ring like Saturn that was situated exactly in a plane with
equator. If we could extend the edge of the ring out into deep space until it sliced through
all the visible sky, it would be much like the imaginary circle we call the celestial
equator. For us in Oklahoma (at latitude 35o) the celestial equator is not in the same
plane as the horizon circle. (Where on earth would the celestial equator and the horizon
circle be the same?)
NOTE: Polaris will
always be at an altitude
equal to the latitude of
the place where it is
being observed. Can you
reason why this is true?
We would see the celestial equator as an arc that traces from due East to due West on the
horizon circle and reaches meridian transit at an altitude of 55o. (How can you calculate
the meridian transit of the celestial equator from the observer’s earth latitude?)
Just as we measured horizontal distance on the horizon circle, we can make a similar
measurement on the celestial equator. Instead of azimuth, however, we call this
measurement right ascension. And instead of using degrees as units, right ascension
divides the celestial equator circle into hours and minutes. Tracing one complete circle on
the celestial equator gives 24 hours. Each 1/24th division (1 hour) of this circle may be
divided into 60 smaller units (minutes).
Vertical distance in this system is called declination. We use degrees just as with
altitude, except assigning 0o to the celestial equator instead of the horizon circle. Also,
degrees may be negative values if we count below the celestial equator.
The Ecliptic: Another way to get your bearings in space.
Although the sun is too bright for us to observe stars in the daytime, stars are still there. If
the sun were smaller and dimmer, we would see it along with the other stars.
Interestingly, if we observed the position of the sun at the same time over consecutive
days, it would be at a slightly different location each day. In fact, it would appear to move
in a straight line (relative to the other stars) about a degree farther to the east each day.
The imaginary line that this would trace over a year-long period is known as the ecliptic.
The phenomenon is caused by the earth’s 365-day revolution around the sun in a plane
along with the other planets. We divide the ecliptic circle into units of months and days.
(365 total, so these units are a tiny bit smaller than the 360 degrees in a circle.) For
example, the location “March 5” on the ecliptic is the position we find the sun on the date
March 5th.
Remember, the sun does not actually move on the ecliptic. It is our annual revolution
around the sun in Earth’s orbit that makes the sun seem to shift to a different background
of stars.
Star Patterns
Stars can be named and grouped into recognizable
clusters or constellations. These constellation patterns
are arbitrarily imposed upon the stars in honor of
particular characters or stories, not because the pattern
resembles the character in form. Eighty-eight
constellations are officially recognized today, and many
of these are of ancient origin. Some “famous” stars
have names, but a more modern system of naming gives
names based on relative brightness within the
constellation. Using the Greek alphabet, the brightest
star is alpha, the second-brightest beta, the thirdbrightest gamma, and so forth.
The letter of the Greek alphabet that indicates a star’s brightness is conjoined
with the Latin genitive form of the name of the constellation (e.g., often changing “us” to
“i”). For example, the brightest star of the constellation Centaurus is alpha-Centauri. The
hunter Orion is one of the most easily spotted constellations, especially in winter, and is
visible from every inhabited part of the globe. His belt of three bright stars in a line is
quite distinctive (and lies nearly on the celestial equator).
Orion’s right shoulder is Betelgeuse (pronounced roughly “beetle-juice”), a red giant that
is one of the largest stars in the sky (capable of being discerned as an actual disk rather
than a point of light with large telescopes; if Betelgeuse were placed at the position of our
Sun, the Earth and Mars would fall well inside its surface). Orion the hunter appropriately
faces the red eye (the star Aldebaran) of the adjacent bull (the constellation Taurus).
Orion’s dog (the constellation Canis Major) walks at his side, containing the brightest star
in the sky (Sirius).
Orion’s lower right foot (Rigel) is a bright bluish-white star at a vertex of the so-called
“Winter Hexagon.” The Winter Hexagon, though not a constellation, conveniently
connects six bright stars forming a hexagon: Rigel, Sirius, Procyon, Pollux, Capella,
Aldebaran as you can see on your star maps. Each of these bright stars is part of a
constellation. Taurus the bull contains the bright cluster of six or seven naked-eye stars
called the Hyades. This cluster is very close to another familiar cluster known as the
Pleiades.
In Egyptian mythology, Orion was the abode of Osiris, the mythical pharaoh who
invented the arts of agriculture before being slain by his animal-headed brother, Set.
Osiris conquered death and, once resurrected, came to reside in Orion (his wife, Isis,
dwelling near Sirius). Jewish tradition eventually identified Orion with Nimrod, the
“mighty hunter before the Lord” spoken of in early Genesis. To the ancient Syrians,
Arabians, and even Ptolemy, Orion was the Giant; to the Chinese Orion was (with
Taurus) the White Tiger. Or, from Ireland, a modern tale of how St. Patrick sent him to
hunt in the heavens begins…
O’Ryan was a man of might
Whin Ireland was a nation,
But poachin’ was his heart’s delight
And constant occupation....
Charles G. Halpine
In the summer sky you may see another collection of constellations called the Summer
Triangle. At about 10:00 pm it is almost overhead. The triangle is made of the brighter
stars Vega (in the constellation Lyra the Lyre), Deneb (Cygnus the Swan), and Altair
(Aquila the Eagle).
Annual (Zodiacal) Motion: Slower than Daily Motion
In addition to the changing appearances that are repeated every day (diurnal motions),
some celestial bodies (planets, moon, sun, but not the stars) change their relative
positions in patterns that become apparent only when considered over longer periods of
time such as a week, month, year, thousands of years, or longer. Such changes are
collectively known as zodiacal motion (or annual motion). The term “zodiacal” derives
from the “zodiac” constellations through which the celestial objects of interest primarily
appear to move).
We often hear that the sun rises in the east and sets in the west. That’s not completely
true. At any particular latitude, we see the sun rising and setting at slightly different
positions on the horizon each day. The sun traces its rising and setting positions on either
side of due east (sunrise) and due west (sunset) along the horizon in an annual pattern that
is exactly repeatable and that recurs seasonally. It will rise and set on due east and west
only two times per year (on the equinoxes). Thus, the sun provides a reliable and
convenient calendar, like a pendulum oscillating around the points due east and due west.
This solar calendar was as significant for ancient cultures as the cycles of day to night
and the lunar phases. The average American today, though ignorant of the term “zodiacal
motion” knows that the Sun appears to go through a cycle that is the basis of the “year” in
our calendar. For example, an ancient Chinese explanation of this pattern attributed the
motion to the Earth, as if the Sun were rising each day at the same place but the Earth
itself slowly sliding up and down along a north-south line.
At Stonehenge (Britain), Woodhenge (Cahokia, Illinois), or other stone circles (e.g.,
Medicine Wheel, Wyoming), seasonal time intervals were charted by arranging sunrise
markers sighted toward the eastern horizon, running north or south of due east. Sunrises
and sunsets reach their extreme northerly and southerly positions on the solstices; and
occur due east and due west on the equinoxes (note that Stonehenge is at a greater
northern latitude than Shawnee and thus the resulting azimuth range for Sun rises is much
greater than in Shawnee).
Planets….
Even the earliest observers noticed that some of the points of light in the night sky didn’t
seem to “obey the rules”. Instead of maintaining fixed positions with other stars in a
constellation, these renegades would seem to wander through the night sky, gradually
shifting positions relative to the other stars. Based on the Greek word for “wanderer”,
these planets seemed very mysterious and caused much speculation in astronomy,
physics, and even theology.
Our solar system is composed of the sun and many objects that are in orbit around it.
According to a 2006 ruling by the International Astronomical Union, a planet is now
narrowly defined as a celestial body that 1) orbits the sun, 2) has enough mass to pull
itself into a sphere, and 3) has enough gravity to clear a space around itself of space
debris. This rule excludes our beloved Pluto on criterion #2. Most scientists now consider
Pluto to be another asteroid or comet, although some categorize it along with other large
asteroids as a dwarf planet.
The four inner planets, those with orbits closest to the sun (Mercury, Venus, Earth,
Mars), are “terrestrial”, meaning that they have solid, rocky crusts. The four outer
planets (Jupiter, Saturn, Uranus, Neptune) are “gas giants”, perhaps not having any solid
layers. Thousands of asteroids in a belt between Mars and Jupiter may be the remains of
an exploded planet, or one that never coalesced into a sphere. All planets revolve around
the sun in the same direction and in approximately the same plane.
The Other Planets, In a Few Words…
Mercury - Smaller than Earth. HOT. No atmosphere or moons.
Venus - About the size of Earth. Covered in poisonous clouds. Hot. No moons.
Mars - Half the size of Earth. Thin atmosphere. Hot and cold. 2 moons.
Jupiter - Cold gas mass,10x bigger than Earth. Large red hurricane on surface. 66 moons.
Saturn - Cold gas mass, 9x bigger than Earth. Rings around equator, 62 moons.
Uranus - Cold gas mass, 4x bigger than Earth. Rings, 27moons.
Neptune - Cold gas mass, 3x bigger than Earth. Rings, 13 moons.
Pluto - (dwarf planet) 5 moons.
Ceres - (dwarf planet) in the asteroid belt between Mars and Jupiter. No moons.
Eris, Haumea, Makemake - (dwarf planets) farther out than Pluto. Some moons.
Since all of the planets have orbits in about the same plane as Earth’s, and the sun
obviously lies on that plane, we would expect to see them somewhere near the ecliptic.
That is indeed the case. Mercury, Venus, Mars, Jupiter, and Saturn are close enough (or
large enough) to be seen with the naked eye on or near the ecliptic.
Now stretch you 3D brain and picture this. Mercury and Venus are in orbits smaller than
that of Earth and take less time to complete their respective orbits. The other planets are
in larger orbits and take progressively more time to complete their revolutions the farther
they are from the sun (Neptune takes 184 Earth years to complete its revolution). Viewed
from Earth, it would appear that we were constantly passing and being passed by other
planets depending on where we all were in our revolutions. Early astronomers were
aware of this phenomenon, and were quite confused by it. Over a period of nights a
planet may appear to pause, then go backward in its path on the ecliptic. This is known as
retrograde.
The Moon…
Many planets in our solar system have natural satellites that revolve around them just as
planets revolve around the sun. While most planets’ moons are relative small compared
with its planet, Earth’s moon (named Luna) is almost one fourth the size of Earth. It
revolves in a synchronous orbit, which results in the same side always facing us, and the
orbit is in about the same plane as Earth’s orbit around the sun.
People have watched and wondered about the moon for millennia, but from 1969-1977,
US missions to the moon confirmed what most scientists have known for some time—
Luna is a barren, rocky place with no atmosphere and very low gravity. Volcanoes were
active at one time, causing the dark flat regions once mistaken believed to be oceans.
They are still called maria (“seas” in Latin). The moon is also pocked with many impact
craters since it has no atmosphere to burn incoming meteors.
From Earth we see the moon go through phases. These are simply the way we perceive
the lighted and dark sides of the moon according to positions of Earth, the moon, and the
sun. It takes about 29 days to cycle through all of the phases, giving us the calendar
designation of a month (moonth?)
Let’s assume the start of the cycle is shown below (distances and sizes are not accurate):
This arrangement is called new moon. We don’t see the moon in this phase because we
are looking at the unlit side. Also, the sun is in the same line-of-sight, so it would be too
bright to see anyway. In about seven days, the moon will have moved one fourth of its
orbit around Earth as shown below.
In the beginning of this 2nd quarter, we can see half of the lit side and half of the unlit
side (a half moon we say). As this 2nd quarter phase continues, the moon will eventually
(about 14 days from new moon) reach halfway around its orbit. Now the moon is on the
opposite side as the sun, so from Earth we see the completely illuminated side.
This marks the beginning of 3rd quarter and is known as a full moon. Sometimes during
this phase, when the moon is exactly in the Earth’s shadow, we see a phenomenon known
as a lunar eclipse. The moon will continue in its orbit to 4th quarter (with a half moon
again) and finally back to new moon and a new cycle.
Obviously, from earth we see these phases as a continuous progression from night to
night. From the invisible new moon, we would begin to see a small sliver of the lighted
side (crescent moon) that would wax (grow) through 1st and 2nd quarters, then wane
(shrink) from full moon to gibbous (a little less than full) and finally to new moon again.
Terms…
Altitude: The vertical angle of a celestial object from where you are observing it.
Altitude is expressed in degrees with 0o at the horizon line and 90o at zenith.
Asterism: A smaller group of stars within a constellation.
Azimuth: A measuring scale set on the horizon circle that is divided into 360 degrees
with 0o aligned with due North. (east = 90°, south = 180° and west = 270°)
Celestial Equator: The imaginary extension of the earth’s equator into space.
Conjunction: The event when two celestial objects are at the same place (or close) on the
ecliptic.
Constellation:
Daily (Diurnal) Motion: The circular motion of the sky around Polaris.
Declination: Vertical measure on the celestial equator.
Eclipse: A form of occultation in which the moon blocks the sun (solar eclipse) or the
earth’s shadow covers the moon (lunar eclipse).
Ecliptic: The circle formed by plotting the position of the sun against the stars for one
year. It also corresponds to the plane of the planets’ orbits around the sun.
Horizon: Simply, where sky meets earth, at an altitude of about 0o depending on hills,
trees, buildings, etc.
Meridian: An arc going from due south to due north running through one’s zenith; the
meridian divides the sky in half (making eastern and western halves).
Meridian transit: The moment of time in a 24-hour period in which a celestial body
appears to reaches its maximum height, or lies on the meridian. This occurs at roughly
daily intervals when the body crosses from the eastern half of the sky into the western
sky.
Occultation: A special type of conjunction in which one object completely covers
another one from our perspective.
Opposition: The event when two celestial objects are exactly opposite (180o apart) on the
ecliptic.
Retrograde: the seemingly backward motion of a planet along the ecliptic due to earth’s
relative position.
Right Ascension: Horizontal measure on the celestially equator.
Rising/Setting time: The moment when an object rises/sets relative to one’s horizon.
Zenith: The point of sky directly over one’s head, or an altitude of 90o.
Zodiacal Motion (annual motion): The motion of the sun, moon and planets along the
ecliptic.