What’s Up There ???

The first step in exploring the cosmos is to consider what can be seen from Earth’s surface, using only your eyes. After all, until about 1610 AD, the only optical aid for viewing the heavens was just our two eyes.

So lets pose the question:

What objects, patterns, and motions define the sky from the perspective of Earth-bound observers, using only their eyes ??

THE OBVIOUS

The day / night cycle. The cycle repeats every 24 hours, with varying amounts of light and dark during the course of a year.

During the day, only three “objects” are visible - the Sun, the Moon and the planet Venus (if you know exactly where to look).

At night, multitudes of bright points-of-light (how many ?), are visible, known as stars. The stars form fixed patterns on the sky, although the entire star pattern appears to rotate from east to west during the night.

There are7 bright objects, which “move” around the sky, with respect to the stars - the Sun, Moon and 5 planets. All but the Sun vary in brightness.

Also visible is a faint, white band of light arcing across the entire sky, known as the Milky Way.

We also occasionally observe unpredictable, transient objects in the sky; objects which appear, then disappear sometime later. These objects have been termed meteors, comets, and “new” stars.

Lets’ look at each of these:

A. THE FIXED STARS:

  • Stars are visible with a rangeof brightness, they do not all appear the same.
  • Grouped into constellations: fixed patterns of stars. All regions of the sky contain some stars.
  • The visibility of specific stars & constellations depends on the date, time of day and the observer’s location (your latitude) on the Earth’s surface.
  • In the Northern hemisphere, the star pattern appears to rotate around Polaris, the pole star.

B. THE “WANDERERS”

  • These objects move with respect to the fixed stars; their location changes with time.
  • The Sun & Moon are bright, “resolved” objects; both show a disk and distinct features.
  • The five planets: Mercury, Venus, Mars, Jupiter, and Saturn appear as points of light, brighter than most stars, but showing NO details. Mars appears red in color, Saturn appears yellow, the other three generally white.
  • Motions of all seven objects are predictable (i.e., repeatable), not random. Tracking their motions has occupied many generations of sky watchers from antiquity to the present.

C. TRANSIENT PHENOMENA

These are celestial objects visible at random and unpredictable times. They may last seconds (meteors) to months.

  • Comets – fuzzy stars with tails, sometimes visible for weeks, moving slowly against the background of stars.
  • Meteors – brief flashes of light, also known as “shooting stars”. 5-10 are visible every hour during the night. Sometimes the number of meteors is much larger, up to 100+ per hour.
  • Novae – “new” stars which appear suddenly, then fade away. Often visible for weeks to months, and considered omens of evil back in ancient times.

These objects and patterns were all the information available to humanity from pre-history to the invention of the telescope in 1610 AD.