Unit 1 Part 2 Constellations

Ursa Major

"great bear"

Ursa Minor

"little bear", contains Polaris

There are 88 constellations. Roughly how many of these can be seen at any given time from a single location on a clear night?

44

How many constellations are there?

88 (as designated and approved by the International Astronomical Union in 1922); many of these are based on historically-defined "constellations"

Constellation

A region in the sky. Think of how the borders and labels of countries on an Earth globe divide the surface into distinct areas

Asterism

A shape traced out in the sky by imagining connecting lines between stars

Constellations crossed by the ecliptic

Aries, Leo, Scorpio, Capricorn, Taurus, Virgo, Ophiuchus, Aquarius, Gemini, Libra, Sagittarius, Pisces, Cancer

Why don't we see the constellation Orion at night all throughout the year?

As the Earth orbits the Sun, its nighttime side points to different constellations at different points along its orbit.

The Ecliptic

As the Earth orbits the Sun, the Sun traces out an apparent path across the celestial sphere called the ecliptic; the term ecliptic also applies to the plane in which the Earth orbits the Sun. The Sun seems to move against the background stars as seen from the orbiting Earth. The ecliptic is the path the Sun appears to trace through the stars. (Green line in picture) Also, all of the planets pretty much orbit in the ecliptic plane.

When is the best time for you to see the constellation that is your sign of the Zodiac?

At midnight six months after your birthday

What is in a constellation?

Constellations contain asterisms, as well as many more stars, nebulae, galaxies, and other objects too faint to see; that is not to say that the constellation physically contains all those objects in a small space, just from our viewpoint on Earth, those things can be seen through the constellation.

Polaris

Earth's current 'pole star' or the 'North Star'

Celestial sphere

The celestial sphere is an imaginary sphere of gigantic radius with the earth located at its center. The poles of the celestial sphere are aligned with the poles of the Earth. The celestial equator lies along the celestial sphere in the same plane that includes the Earth's equator.

The Zodiac

The subset of constellations which are crossed by the ecliptic line.

Constellations close to the North pole of Earth do what in relation to the North star?

They move in a circle around the North star, Polaris, in the course of a single night.

What asterism the the 'Big Dipper' located in?

Ursa Major

When are constellations visible?

Which constellations are visible on a given night depends on the time of year and the viewer's location. As the Earth travels around the Sun in its orbit, the Sun blocks view of constellations in its direction. The constellations visible at midnight one night will be behind the Sun at noon half a year later, while those previously obscured by daylight will be prominent in the nighttime sky.

Earth's precession

a change in the orientation of the rotational axis of a rotating body. Cyclical variations in Earth's motions over long times — thousands of years — do cause some drift in the timing of the Sun's passage through the zodiac constellations. Also responsible for other effects, such as the change over time in which star is best considered a "pole star"