ONSTELLATIONS C Altitude Ruler The rotation of the Earth on its axis causes the stars to rise and set each evening. In addition, the orbit of the Earth around the Sun places different regions of the sky in our Horizon night-time view. The PLANISPHERE is an extremely useful tool for finding stars and 10 constellation in the sky, depicting not only what is currently in the sky but it also allows the 20 prediction of the rising and setting times of various celestial objects. 30
THE LAYOUT OF THE PLANISPHERE 40 50 The outer circumference of the dark blue circular disk (which is called the star wheel) you’ll notice that the wheel is divided into the 12 months, and that each month is divided into 60 individual dates. The star wheel rotates about the brass fastener, which represents the 70 North Celestial Pole. The frame of the planisphere has times along the outer edge. 80 Holding the planisphere on the southern corner you'll see "midnight" at the top. Moving Zenith counterclockwise, notice how the hours progress, through 1 AM, 2 AM, and so on through "noon" at the bottom. The hours then proceed through the afternoon and evening (1 PM, 2 PM, etc.) back toward midnight.
Once you have the wheel set properly for the correct time and day, the displayed part represents what you see if you stand with the star and planet locator held directly over your head with the brass fastener toward the north. (Notice that the compass directions are also written on the corners of the frame.) Of course, you don't have to actually stand that way to make use of the Star and Planet Locator--this is just a description to help you understand what is displayed.
LOCATING & IDENTIFYING STARS
Stars are depicted by circles with increasing brightness represented by increasing diameter. The planisphere is designed to be held above your head so that it represents the stars in the sky.
Hold the planisphere by the South horizon corner above your head. Notice that East is to your left and West to your right. If you are looking North, then hold the planisphere by the North horizon. You’ll notice that the East horizon in now on your right and West on your left.
Rotate the inner disk so that May 15 aligns with 8 pm. Slowly rotate the inner disk so that May 15 now aligns with 8 pm, then rotate it to 9 pm, then 10 pm, and so on. What direction (clockwise or counter clockwise) do the stars move? From what horizon do the stars rise (N, S, E, W) and what horizon do they set (N, S, E, W)?
Direction: Counter clockwise Rise: Eastern Set: Western
The meridian is an imaginary line that runs through the North / South horizon, bisecting the sky into East and West horizons. List two constellations with prominent stars that can be found near the meridian at 10 pm on January 15.
Constellation Prominent Star(s) Orion (The Hunter) Betelteuse, Rigel Auriga (The Charioteer) Capella Constellations 1 The stars “rotate” around a central point on the planisphere. What star do all of the stars in the sky rotate around? What constellation is the star located?
Star Polaris (The North Star) Constellation: Ursa Minor (The Little Bear)
ALTITUDE is a measure of the elevation an object is above the horizon. Objects on the horizon have an altitude of 0o, and objects at an altitude of 90o are located directly overhead, or at Zenith.
Place the Altitude Ruler so that the Horizon line is on the northern horizon. Measure the altitude of Polaris above the northern horizon.
Altitude 40o
This planisphere is designed for the night sky for locations with a latitude of 40o N, such as Providence. How would the altitude of Polaris change for locations north of Providence such as Fairbanks, Alaska with a latitude of 65o N? What is the altitude of Polaris in Fairbanks?
As the latitude increases (more northerly) the altitude of Polaris increases. The latitude of the observer is the same as the altitude of Polaris. Fairbanks: 65o
How does the altitude of Polaris change for locations south of Providence, such as Miami, Florida with a latitude of 25o N? What is the altitude of Polaris in Miami?
As the latitude decreases (more southerly) the altitude of Polaris decreases. The latitude of the observer is the same as the altitude of Polaris. Miami: 25o
Where on earth would you find Polaris on the horizon? Where would you find Polaris directly overhead? Horizon: Equator Overhead: North Pole
Located on the meridian, a star or constellation at your Zenith (altitude = 90o) is directly overhead. Vega, a bright star in the constellation Lyra (the Lyre) can be seen in April nearly directly overhead early in the morning before the sun has risen. What time would you see Vega at your Zenith on April 15?
5 am
Capella, in the constellation Auriga can also be found at our Zenith in the winter months. What time would you expect to observe Capella on January 1 at Zenith?
10:30 pm
Constellations that are visible the entire year are called circumpolar constellations. Rotate the planisphere throughout the year and record the names of four circumpolar constellations.
1. Cassioeia (The Queen) 2. Auriga (The Charioteer)
3. Cepheus (The King) 4. Ursa Major (The Big Bear)
5. Draco (The Dragon) 6. Camelopardalis (The Giraffe) (too faint to be seen)
Constellations 2 N AZIMUTH is a compass direction where the object (star, planet, NNW NNE sun or moon) can be found in the sky. Along with altitude, NW NE these two measurements can accurately describe the position of an object at a point in time. WNW ENE Rotate the planisphere so that November 15 aligns with 10 W E pm and find the brightest star in the sky, Sirius, located in the constellation Canis Major. Which direction along the WSW ESE horizon would you look? (Be specific, such as S, SSE, SE, ESE, etc ) SW SE SSW SSE East Southeast (ESE)
S Determine the altitude of Sirius: 0o (at the horizon)
Rotate the disk so that you observe the motion of the stars every month at 9 pm until April 15. Imagine that you walk your dog every night at 9 pm during the winter and spring months and you were to observe Sirius during this period. Would Sirius be found in the same part of the sky throughout the winter? Describe the changing position of Sirius during this interval of time. What motion of the earth is responsible for this observation?
The stars make a slow migration westward throughout the year, rising earlier in the evening each month. Sirius is found in the southeast horizon in the fall. During the winter months it gradually rises earlier each month so that it is found higher in the sky and moving through the southern horizon. Sirius can be found in the southwest horizon in the early spring.
Place the star Sirius in the southeastern horizon. Approximately what date of the year does Sirius make its appearance in this horizon at sunrise (5 am)? This time of the year is referred to as the “dog days of summer”. Date: Early August
Rotate the planisphere to 10 pm on your birthday. What constellations and stars are prominent near the meridian at this time of the year? Date: October 26
Constellations Prominent Stars
Cassiopeia, Pegasus, Aries None
This Red Giant Star is the red eye of the constellation Scorpius (The Scorpion): Antares
This Red Giant is the red eye of “The Bull” in the constellation Taurus: Aldebaran
The “Summer Triangle” is formed by the prominent stars Deneb, Vega, and Altair. What constellations are these stars located?
Cygnus (The Swan) Lyra (The Lyre [harp]) Aquila (The Eagle)
During what month can the Summer Triangle be found around midnight on the meridian?
July
Constellations 3 ASTRONOMY MEETS MYTHOLOGY Locate and identify the following constellations.
The myth of Andromeda claims that her mother, Queen Cassiopeia, was so proud of her daughter's beauty that she boasted it surpassed even that of the nymphs of the sea. Such arrogance offended the gods, who released floods in retribution and threatened to destroy the kingdom. Andromeda was chained to a rock by the sea upon the orders of her father, Cepheus, in the hope that her sacrifice to the sea-monster Cetus would appease the gods of the sea. While awaiting her fearful fate, the princess was heroically rescued by Perseus who exposed to Cetus the decapitated head of Medusa - a sight so horrifying and morbid that the monster turned to stone. Pegasus was born from the union of Medusa and Neptune when Perseus threw Medusa's head into the sea. The winged horse is famed in mythology for creating a fountain of water at Helicon with a stamp of his hoof.
Canis Major and Canis Minor are the dogs of Orion, the hunter, which lies immediately to the north- west. In this guise they are frequently shown as a dogs standing on their hind feet, waiting on the command of its master to spring after Lepus the hare.
THE SUN AND THE SEASONS
Located on the planisphere is a dashed line, labeled the ecliptic. This represents the path the sun makes through the sky during the year against the background stars. Throughout the year the sun’s altitude changes with the seasons. Since the sun rises in the east and sets in the west, the sun reaches a maximum altitude as it passes through the southern sky at a time called solar noon when the sun is at the meridian.
The “equator” line on the Locator is the Celestial Equator, an extension of the Earth’s equator. These two lines intersect twice during the year. Locate the intersections when they appear in the Southern horizon along the meridian. Record the date as they appear in the South horizon window on the Locator. What is the significance of these dates?
March 21 & September 21 The equinox is where the sun is aligned with the Earth’s equator. The Sun can be found directly overhead on the Earth’s equator.
What dates are the ecliptic and equator furthest apart? What is the significance of these dates?
December 21 On the Winter Solstice the Sun (ecliptic) is found south of the Celestial Equator. The Sun can be found directly overhead on the Tropic of Capricorn.
June 21 On the Summer Solstice the Sun (ecliptic) is found north of the Celestial Equator. The sun can be found directly overhead on the Tropic of Cancer.
Rotate the planisphere to measure the altitude of the sun at noon today by placing the date in the Southern window of the plansiphere. Use the Altitude Ruler to measure the altitude of the sun (ecliptic) above the southern horizon.
Repeat the process for the dates in the table below.
Constellations 4 Season / Date Altitude Today / December 1 30o Winter Solstice / December 21 25o Vernal Equinox / March 21 48o Summer Solstice / June 21 71o Autumnal Equinox / September 21 48o
What is the change in altitude of the sun from season to season? What feature of the Earth’s motion which is responsible for this seasonal change? Approximately 23o each season. This is due to the tilt of the Earth’s axis. To determine the azimuth (location at the horizon) and the time for sunrise, rotate the dial so that today’s date is in the center of the Eastern window on the planisphere. The azimuth and time is determined where the sun (ecliptic) intersects the horizon. Use your pencil to form a straight line through this point and Polaris (the grommet) to estimate the azimuth and time for sunrise. The process for sunset is similar to that above except place today’s date in the Western window.
Sunrise Sunrise Sunset Sunset Season / Date Location Time Location Time Today / December 1 ESE 7:00 am WSW 4:45 pm Winter Solstice / December 21 ESE 7:00 am WSW 4:30 pm Vernal Equinox / March 21 E 6:00 am W 6:00 pm Summer Solstice / June 21 ENE 4:00 am WNW 7:00 pm Autumnal Equinox / September 21 E 6:00 am W 6:00 pm
THE PLANETS As you have seen, the dashed line on the planisphere marks the ecliptic. As our Earth moves around in its orbit, the Sun, the Moon, and the planets also appear along this line. The ecliptic therefore represents the plane of the solar system. Of the nine planets, five are easily visible without a telescope. The table on the reverse side of the plansisphere contains the constellation in which a planet can be found for during this year (2011). In the sky, the planets will appear as bright points of light, sometimes much brighter than the surrounding stars. Which planets are visible around 10 PM tonight (December 1)? Which planets are visible around 3 AM?
10 PM Planet Constellation Altitude Azimuth Jupiter Libra 20o ESE
Constellations 5 3 AM Planet Constellation Altitude Azimuth Mars Sagittarius o Southeast 20 Saturn Sagittarius
Examine the chart again and provide an explanation why the planets Jupiter and Saturn tend to remain in a given constellation for several months while Mars can be found in different constellations throughout the year. Jupiter and Saturn are at much greater distances from Earth than Mars. Jupiter and Saturn move much more slowly against the background stars than does Mars. Mars’ orbit is relatively close to Earth’s orbit, therefore from our perspective Mars tends to move more rapidly against the background stars. Why are Mercury and Venus seen only during the morning (MOR) or evening (EVE) hours? From our perspective Mercury and Venus are located close to the Sun. We are only able to observe these planets when the Sun is below the horizon. Therefore, at some times we can only see Mercury and Venus before the Sun has risen in the morning or after the Sun has set in the evening. Neither Mercury nor Venus can be seen late at night.
Constellations 6 THE PLANISPHERE: POST-LAB
1. On May 1st at 8:30 pm what prominent star is about to set in the west?
Sirius (Canis Major) or Aldebaran (Taurus)
2. On May 1st at 11:30 pm what star is near your zenith?
Arcturus (Bootes) “The Heardsman”
3. On Feb 1st at 1:00 am, what is the altitude of Polaris?
40o from the northern horizon
4. What does the ecliptic represent?
The plane of the solar system and the path of the Sun against the background stars. It is the yearly orbit of the Earth that causes the Sun to “move”.
5. Where will the sun rise on December 25? ESE When? 7:00 am
Where will the sun set on December 25? WSW When? 4:45 am
6. Where will the sun rise on July 4? ENE When? 4:30 am
Where will the sun set on July 4? WNW When? 7:30 pm
7. What is the altitude of the sun at noon on December 25? July 4?
December 25: 25o above the southern horizon.
July 4: 70o above the southern horizon.
8. Why are only certain constellations designated as constellations of the Zodiac?
They lie along the ecliptic. The particular birthsign of the Zodiac is determined when the Sun is located in that constellation, not when the constellation is visible in the sky.
9. What sign of the zodiac is your birthday?
October 26 (Scorpius)
10. Approximately what date would you go outside to see this constellation at midnight on the meridian?
Early – mid June
11. What time is Orion in the southern sky today? Two months earlier? Two months later?
6:15 am (October 1) 2:15 am (December 1) 10:00 pm (February 1)
12. What two prominent stars are located in Orion? Where are they located within the constellation?
Rigel (lower right), Betelgeuse (upper left)
13. What time does the star Sirius rise on December 1? What time does it set? How many hours is Sirius visible in the sky on December 1?
9:00 pm (rise) 7:00 am (set) 10 hours above the horizon
Constellations 7