ASTR-101/L Constellations and the Celestial Sphere Study Notes

Constellations and the Celestial Sphere

Introduction to Constellations

  • The sky is divided into constellations, which are rectilinear areas of the sky with agreed-upon boundaries.

  • There are a total of 88 constellations, and no two constellations overlap.

  • These constellations cover every part of the sky.

Constellations vs. Asterisms

Constellations

  • Defined as the official borders of an apparent grouping of stars in the sky.

  • Stars within a constellation are not physically bound to each other nor are they necessarily close to one another in space.

  • Each star’s distance from Earth can vary greatly.

Asterisms

  • Represent recognizable patterns of stars or connect the dots to general shapes, often resembling objects, people, or creatures.

  • Asterisms can exist within constellations.

Visual Representation

  • Constellations are denoted with green lines, while Asterisms are represented with red lines.

  • Examples include:

    • Orion, represented by the implication that the stars within its official boundary may include stars not commonly recognized as part of the asterism of Orion.

    • Big Dipper as an asterism within Ursa Major constellation.

Notable Constellations

Winter Constellations

  • Constellations associated with the winter season include:

    • Gemini

    • Canis Minor

    • Auriga

    • Monoceros

    • Camelopardalis

    • Taurus

    • Orion

    • Canis Major

    • Lepus

    • Columba

    • Eridanus

    • Fornax

The Winter Triangle

  • Formed by the stars Procyon, Sirius, and Betelgeuse.

Imagery and Navigation

  • Utilizing stars, such as following the two stars on the edge of the Big Dipper to locate Polaris (the North Star).

  • Understanding how the Little Dipper (Ursa Minor) and other constellations relate to navigation.

Seasonal Constellations

Spring Constellations

  • Include:

    • Ursa Minor

    • Boötes

    • Canes Venatici

    • Coma

    • Ursa Major

    • Lynx

    • Leo Minor

    • Cancer

    • Berenices

    • Leo

    • Sextans

    • Virgo

    • Crater

    • Hydra

    • Corvus

Summer Constellations

  • Notable constellations:

    • Draco

    • Cygnus

    • Lyra

    • Vulpecula

    • Delphinus

    • Hercules

    • Sagitta

    • Equuleus

    • Aquila

    • Capricornus

    • Scutum

    • Corvus

    • Serpens

    • Ophiuchus

    • Sagittarius

    • Scorpius

Autumn Constellations

  • Feature:

    • Perseus

    • Aries

    • Cassiopeia

    • Triangulum

    • Andromeda

    • Cetus

    • Pisces

    • Pegasus

    • Lacerta

    • Aquarius

    • Cepheus

    • Sculptor

    • Piscis Austrinus

Ecliptic & Motion

Celestial Sphere Overview

  • The celestial sphere is a model that helps to understand observed motions of stars. It embeds all stars onto an imaginary sphere with a large radius, rotating while the Earth remains motionless.

  • This model is incorrect but serves adequately to depict how stars move across the sky.

Earth's Motion

  • Rotation: One complete spin on its axis that takes 24 hours.

  • Revolution: One orbit around the Sun that takes 365 days.

  • The rotation leads to diurnal motion of the stars (daily motion from east to west).

Proof of Earth's Rotation

  • The Foucault Pendulum, designed by Léon Foucault in 1851, demonstrates Earth's rotation. The plane of oscillation remains static while the Earth rotates underneath it.

Celestial Poles and Observational Astronomy

Celestial Poles

  • The North Celestial Pole (NCP) and South Celestial Pole (SCP) are the theoretical points where the earth’s axis intersects the celestial sphere.

  • Polaris, the North Star, remains stationary in the sky, serving as a navigational tool, while there is no equivalent South Star.

Visibility and Horizon

  • The celestial sphere is divided into visible and not visible areas by the horizon.

  • The horizon is unique to each observer, with cardinal directions determined around the horizon.

  • Rising and setting of celestial objects, like stars, depend on the observer’s location.

Circumpolar Stars

  • These stars never set below the horizon for given locations, and their visibility changes based on the observer's latitude.

  • The concept of circumpolar stars is especially relevant for observers in the northern and southern hemispheres.

Ecliptic and Zodiac

The Ecliptic

  • The ecliptic is the apparent annual path of the Sun through the stars, marked by the Zodiac constellations.

  • The ecliptic differs from the daily motion and is observed throughout the year.

Zodiac Constellations

  • As the Earth orbits the Sun, the visible stars change, and the zodiac is comprised of 12 constellations the Sun appears to travel through during a year.

  • The zodiac is commonly referenced in astrological contexts, with varying lengths of time spent by the Sun in each constellation, averaging around 28 days.