Discovering the Night Sky

How much time does it take for a full cycle of moon phases?

A full cycle of phases takes 29.5 days.

What is a lunar eclipse?

A lunar eclipse occurs when the Moon passes through the Earth's shadow. This only occurs when the Sun, Earth, and Moon are in complete alignment at full Moon. The Moon darkens to a rust color.

What is a solar eclipse?

A solar eclipse occurs when the Moon's shadow moves over the Earth's surface. The Moon moves in front of the Sun at a new Moon.

What is the ecliptic?

A straight path traced by the Sun on the celestial sphere. It intersects with Earth's drawn axis at two equinoxes (vernal equinox and autumnal equinox).

What is the difference between an asterism and a constellation?

An asterism is a pattern of bright stars (i.e. The Big Dipper and Orion), informally called constellations. Constellations are entire regions of the sky (88) that contain asterisms (they make up a quilt-like pattern).

Which coordinate system (Azimuth/Altitude or Right Ascension/Declination) is constant for all observers?

Any star has the same Right Ascension/Declination across the globe. It is constant for all observers because the 0 hours at the prime meridian is the same for everyone.

Why does the moon have phases?

As the Moon orbits Earth, it's position changes across the sky. Therefore, it's position in accordance to the Sun also changes. The Moon has phases because of the different amounts of sunlight hitting its surface.

What does it mean when a star is circumpolar?

At a certain point on Earth, a certain star never sets below the horizon.

Define azimuth and altitude and show how they can be used to locate stars in the sky.

Azimuth - the angle right of north on the reason, can range from 0 degrees to 360 degrees Altitude - the angle straight up from the horizon to YOUR zenith (there's a different one everywhere)

How are constellations useful for astronomers?

By groupings stars into different sections of the sky, they become easier to locate. You may know that a certain star falls into one of the 88 constellations, and pick it out that way.

What is the celestial equator, and how is it related to Earth's equator? How are the north and south celestial poles related to Earth's axis of rotation?

It is directly along the Earth's geographical equator and splits the sky into northern and southern hemispheres. The celestial poles are where an imagined straight line drawn through both geographical poles intersect with the celestial sphere. The sky appears to rotate around these specific points, especially when viewed at night.

Why does the Earth have seasons?

It is, in large part, due stop the 23 1/3 degree tilt that Earth has on its axis. At certain points o the year, either the northern or southern hemisphere is pointed at the Sun, allowing certain times of year to be more directly contacted by sunlight (leading to higher/lower temperatures).

By about how many degrees does the Sun move along the ecliptic each day? Explain why.

It only moves less than 1 degree. This is due to the 365.24 days in a year, and only 360 degrees on a circle.

How much time does it take for the moon to circle the Earth?

It takes the Moon 27.3 days to completely circle Earth (known as the sidereal month).

When is the Earth closest to the Sun?

January 3 (the dead of winter in the northern hemisphere).

Which constellations is not part of the astrological zodiac?

Ophiuchus - the Serpent Holder.

What are Right Ascension and Declination?

Right Ascension (R.A.) - the equivalent of longitude (side-to-side); measured around the celestial equator, 0-24 hours (starts horizontally at the prime meridian) Declination (Dec.) - the equivalent of latitude (up and down); it is measured north or south of the celestial equator from 0 degrees to 90 degrees

What is the zodiac?

The 13 individual constellations through which the Sun moves throughout the year as it travels along the eclipse. Pisces is the center of a set of constellations that will only be visible at night in winter, and Virgo in summer.

Through how many constellations does the Sun move every year? Explain why.

The Sun moves through 13 zodiac constellations per year, all spread across the celestial sphere to fit into 12 months. However, there are only 12 commonly known zodiacs as a result of 12 months.

Through how many constellations does the Sun move every day? Explain why.

The Sun moves through less than one constellation a day. This is a result of there being 13 zodiac constellations having to fit in rotation throughout a full year (12 months).

What is the celestial sphere, and why is this ancient concept still useful today?

The celestial sphere is a sky map with an applied coordinate system, just like longitude and latitude.

What is the celestial sphere?

The celestial sphere is a sky map with an applied coordinate system, just like longitude and latitude. If you can navigate it, you can locate many of the more noticeable stars in the night sky. This is because the constellations are somewhat fixed on the sphere (their so far away that we don't notice any change in their positions in our lifetimes).

What is the ecliptic, and why is it tilted with respect to the celestial equator?

The straight path the Sun traces on the celestial sphere, as viewed from Earth. It is tilted with respect to the celestial equator because Earth's axis is tilted towards the Sun at a 23.5 degree angle.

Name the 3 stars of the Summer Triangle and what constellations they are found.

The threes stars in the summer triangle are Vega, Deneb, and Altair. Vega is found in Lyra, Deneb in Cygnus, and Altair in Aquila.

What are the vernal and autumnal equinoxes? What are the summer and winter solstices? How are these four events related to the ecliptic and the celestial equator?

The vernal equinox is at right ascension 0h and occurs on March 20 every year. The autumnal equinox is at right ascension 12h on September 22 each year. Vernal marks the beginning of spring, while autumnal marks the beginning of fall. The summer solstice is the longest day of the year, with the highest altitude of the sun at noon. The winter solstice is the shortest day of the year, with the lowest altitude of the Sun at noon. The smallest difference in the azimuths between sunrise and sunset is during the summer solstice, while the winter solstice has the largest. These markings all cross the celestial equator in relation to the movement of the Sun and the stars, and the changing of the seasons.

How many constellations are in the zodiac?

There are 13 zodiac constellations (#13 is Ophiuchus - the Serpent Holder).

Describe how 4 different stars can be found using the Big Dipper.

You can imagine the Big Dipper to swing down onto the asterism Leo's head (brightest star, Regulus). You can also follow the arc of the Big Dipper's (Ursa Major) handle to cross, first, Arcturus in Bootes, and then Spica in Virgo. Trace a line from one of the Big Dipper's pointer stars to Polaris in the Little Dipper (Ursa Minor).

Where is the zenith?

a. directly overhead

Which of the following lies on the celestial sphere directly over Earth's equator?

b. celestial equator

How many zodiac constellations are there?

c. 13

The length of time it takes Earth to orbit the Sun is:

d. a year

Describe the 3 types of lunar eclipses.

penumbral eclipse: a lunar eclipse in which the Moon passes solely through the Earth's penumbra. partial eclipse: a lunar eclipse in which the Moon does not appear fully covered. This occurs when only a part of the Moon passes through the umbra. total eclipse: a lunar eclipse in which the Moon passes entirely into the Earth's umbra.

Describe the 3 types of solar eclipses.

total eclipse: a solar eclipse in which the Moon completely covers the Sun. Only the tip of the Moon's umbra reaches the Earth's surface, which traces along the eclipse path as the Earth rotates. The solar corona (hot gases around the Sun) is visible during this eclipse. partial eclipse: a solar eclipse in which the Sun does not appear fully covered. This occurs when only the Moon's penumbra covers the Earth's surface (during a solar eclipse). Only a present shape of the Sun is visible. annular eclipse: a solar eclipse in which the Moon is too far from Earth to appear as a total eclipse. The Moon appears to small to cover the Sun completely. A thin ring of light (or "annulus") appear around the Moon at the time of a mid-eclipse.