The Beginnings of Astronomy and Physics
People in primitive cultures relied on the natural world to provide their subsistence. They noted seasons of food growth and migrations of animals correlated with repetitious patterns in nature. Some of the most obvious patterns were in the sky. Vocabulary was developed in each language as labels for each pattern (a number of useful English terms are defined below):
While some correlations such as the tides, growing seasons and animal migrations still are believed valid today, Astrology has lost much of its appeal because most earthly events are now believed to have earthly rather than astronomical causes. Yet Astronomy. the study of events and motions in the heavens, remains significant today for two different reasons:
- The Sun repeatedly rose and arched the sky before setting. The direction of average rising was called East, and that of average setting called West. After a period of darkness the process was repeated in a time period we call a day.
- The dimmer Moon followed a similar pattern but also changed shape and time of rising over 29 or 30 days.
- The Sun's path across the sky moved to the side (north and south) over a period of about 365 days or between 12 and 13 Moon patterns called months. The sun's period, called a year, closely matched the time between growing seasons.
- At night stars were visible in set patterns called constellations.
- Similar to the Sun and Moon, most of the stars also moved across the sky nightly. The motion of the constellations was the same as if the fixed stars were attached to a sphere which rotated around the earth nightly from East to West. The equator around this imaginary sphere is called the celestial equator. Those stars near the axes of that sphere moved in circles, counterclockwise in the North and clockwise in the South.
- The Sun moves slightly more slowly across the sky than the stars, falling behind the rotation of the stars in the same period (a year) as its North-South-North-South motion. The Moon falls one rotation behind the stars in a sidereal month which is a couple days less than a month. Both the Sun and Moon also change slightly in size.
- Besides the Sun and Moon, five star like objects also wandered through the stars, each falling behind the rotation of the stars in an unique period of time. These seven wandering heavenly objects, known as planets, all approximately followed the equator of the celestial sphere but wandered a few degrees north or south similar to the Sun.
Ancient symbols for the seven wandering planets
- The five dimmest planets also occasionally reversed their direction of motion from their normal slow eastward path amongst the stars, occasionally retrograding across the sky westward faster than the stars. These planets appeared a little brighter when they retrograde.
- Several correlations between these patterns in the heavens and earthly occurrences such as daylight, the tides, the growing seasons and the migration of birds and other animals suggested possible causal relations. The practice of predicting events on earth and the fortunes of men based on these correlations became known as Astrology.
- The well documented historical development of Astronomy provides one of the clearest illustrations how the process of science works.
- The information gathered by Astronomers is providing clues leading to a fundamental understanding of the universe and how we came to be here.
Since most objects in the sky appear to have relatively fixed positions or motions, even the earliest Astronomers realized the value of mapping their locations. Mapping the heavens has the same difficulties as mapping locations on the spherical earth. One common procedure is to flatten what is seen overhead into a flat circle. Becoming familiar with these maps will be valuable for both understanding motions in the heavens, and locating interesting objects in the sky.
The surface of the earth blocks half of our current view of the objects in the sky. Hills, buildings, trees and clouds may block much more of our view. The bright light of the Sun, the Moon, and human generated lights typically over powers the view of dimmer objects in the sky. But the sky maps can help us understand and perhaps view objects visible in the sky under the best of viewing conditions. Sky maps are available in books, magazines, and on the WEB.
To obtain a sky map to view, use this link to John Walker's Your Sky and open in a new window or tab a Sky Map for a viewing location of your choice. Select a nearby city from the selection provided. You might stagger the position of the two windows on your screen so you can alternate between the following directions and the Sky Map.
- The white dots represent stars. Larger dots indicating greater brightness. Constellation names are given in yellow (at night, or white during the day). Names of the most prominent stars are in pink. According to the map what constellation should be at you zenith (directly overhead, the center of the map)?
- Locate the blue arc which represents the celestial equator (green during the day). The graduations (with labels such as 22h) represent the amount the celestial sphere appears to rotate every hour.
- Locate the purple arc which represents the path of the Sun through the stars, called the ecliptic. It is inclined about 23°26' compared to the celestial equator. If it is currently daylight, an icon using concentric yellow circles will represent the sun's location. (If you want a map for viewing at a different time, scroll down the page and increase or decrease the universal time by a number of hours then click Update. The format is year-month-day hour:minute:second: example 2013-04-10 22:21:59) Other icons may lay along the ecliptic. They might represent the moon or some of the other planets which also wander among the stars but stay close to the ecliptic. The sun falls behind the stars about one degree a day, completing a complete rotation of 360° in 365 days. Thus is the ecliptic has a scale labelled in degrees of rotation.
- The Sun's path crosses the celestial equator twice a year These events are know as the vernal (OFr verd = green, L viridis = green, L ver = Spring) equinox and autumnal equinox and approximately represent when day and night are equally long. (Refraction of sunlight at dawn and dusk may make daylight last slightly longer.) These dates have become the traditional beginning of Spring and Fall seasons. Knowing the current date and the degree scale on the ecliptic, determine when the equinox visible on the sky map will (or did) occur.
- At what date does the Sun reach its maximum north or south (maximum distance from celestial equator, 90° from the Equinox)? These events are known as the Summer and Winter Solstices and mark the beginning of the Summer and Winter seasons.
- Using the degree scale, estimate how many degrees the Sun moves north and south of the celestial equator?
- If you lived along the earth's equator, the Sun would be directly overhead at noon on an Equinox. But if you lived in Boston (42° north of the equator) how many degrees would the Sun be below overhead at noon on an Equinox? How many degrees would the Sun be above your horizon?
- How many degrees would the Sun be from overhead in Boston on the Winter Solstice? How many degrees would the Sun be above the horizon? Sunlight strikes Boston most obliquely at the Winter Solstice. And because the arc of the Sun's path is lower in the sky, dawn is later and dusk earlier, making the length of the day shortest. Humans have long understood that cold Winter is the consequence of reduced sunlight.
- Scroll down the sky map to read your laditide. How many degrees would the Sun be from overhead where you are on the Winter Solstice? How many degrees would the Sun be above your horizon?
- Larger dots indicate brighter stars. Locate the brightest star in the constellation at your zenith. The brightest stars of each constellation are designated by Greek letters often called
Bayer letters after Johann Bayer who first identified stars this way in his Uranometria of 1603. Thus the brightest star in a constellation is generically named
α (alpha) followed by the constellation name. Bright stars also often have common names (shown in pink on this star map). What is its common name of the brightest star near your zenith?
- Nearly two centuries ago a number of fuzzy objects were observed which when eventually viewed through large telescopes were recognized to be large collections of stars. We now called these galaxies. Charles Messier (b1730, d1817, age 40 portrait at right→) catalogued about 100 fuzzy objects that didn't move compared to the stars so that observers using small telescopes would not confuse them with comets. To located the nearest example on the Sky Map, use this link to sky objects, then select Constellations, then Andromeda. Locate the galaxy (the icon looks like a whirling ball) in the middle of the constellation Andromeda. What is its Messier catalog number? Use the following link to view a variety of images of Andromeda and read about recent discoveries.
- More recent objects have been listed in the New General Catalog (NGC).
- We humans generally have diverse interests! Now that you have been introduced to this sky map and a bit of basic astronomy, feel free to brows further and investigate other aspects of the sky map and the astronomy it describes.
- If you live in an area with enough night darkness you might try to match constellations on the map with those in the sky. You might try to locate a galaxy using the sky map, then view it in the night sky with binoculars.
Note that the earliest Astronomy involved observations, mapping and labeling such as we have been studying here. These types of activities are fundamental for all science. Observations and their organization are found at the beginning of every science. They also are found at the core of every mature science.
Finally, record your procedures, measurements, and findings in your journal. If you need course credit, use your observations recorded in your journal to construct a technical report