Key Ideas and Terms

1-1 Astronomy is both an ancient cultural practice and a cutting-edge science

• All scientific methods use observations or experiments to pursue evidence to answer questions about the natural world, but they use a variety of approaches and no single set of required steps.
• A theory is a body of related observations that can be pieced together into a comprehensive, self-consistent explanatory description of how nature works.
• Science is a creative and innovative human enterprise embarked upon by a community that agrees to an informal code of ethics.
• Technology has vastly improved our ability to learn about the universe, resulting in a scientific knowledge base that rapidly changes as new data and theories appear.

1-2 The stars are grouped by constellations

• Ancient peoples imagined groupings of stars to be recognizable shapes called asterisms.
• In modern science, the entire sky is divided into 88 regions, each of which is a constellation.

1-3 All of the observed celestial motions can be described if our planet Earth spins once each day while it orbits around our Sun each year

• Earth completes one full rotation from west to east every 24 hours, which explains why the stars, Sun, and Moon all appear to rise in the east, move across the sky, and set in the west.
• Earth makes a complete orbit of the Sun each year, which is why the constellations appear to shift positions in the sky.
• Projecting Earth’s equator and poles into space onto an imaginary celestial sphere surrounding Earth, we label the celestial equator, the north celestial pole, and the south celestial pole.
• The rotation of this imaginary celestial sphere carries stars across the sky in paths parallel to the celestial equator, and the angle of a star’s path across the sky depends on the latitude of the observer.

• The point in the sky directly overhead an observer anywhere on Earth is called that observer’s zenith.
• Circumpolar stars are stars sufficiently near the north celestial pole so that they revolve around the pole, never rising or setting beneath the horizon.

1-4 The Sun appears to change position over the day and throughout the year, and these changes result in Earth’s seasons

• Earth’s axis of rotation is 23½° away from perpendicular.
• When either one of Earth’s hemispheres is tilted toward the Sun, the days are long and sunlight strikes the ground at a nearly perpendicular angle, heating the ground efficiently, resulting in that hemisphere’s summer season.
• The circular path that the Sun appears to trace out against the background of stars over the course of a year is called the ecliptic, and the plane of this path is the ecliptic plane.
• The ecliptic and the celestial equator intersect at only two points, each called an equinox, on exactly opposite sides of the celestial sphere.
• When the Sun appears to cover either of these points—on about March 21 is the March equinox and on about September 22 is the September equinox—day and night are each 12 hours long at all locations on Earth.
• On the ecliptic between the March and September equinoxes lie the northern solstice, the point farthest north that the Sun reaches, and the southern solstice, the point farthest south.
• On the southern solstice at the Arctic Circle, the Sun is below the horizon for 24 continuous hours, and at the Antarctic Circle, there is 24 hours of continuous daylight, while on the northern solstice this is reversed.
• Between the Tropic of Capricorn at 23½° south latitude and the Tropic of Cancer at 23½° north latitude, the Sun is directly overhead—that is, at the zenith—at high noon at least one day a year.

1-5 The Moon appears to change its position in the sky hourly and its phase throughout each month

• Our Moon orbits Earth approximately every four weeks, while the whole Earth-Moon system orbits the Sun.
• Our Moon shines not from light of its own, but rather by reflecting sunlight. We only see the illuminated portion.
• The different amounts of the illuminated half of the Moon that we see over the course of its orbit are called lunar phases.
• The phases of the Moon are new moon, waxing crescent moon, first quarter moon, waxing gibbous moon, full moon, waning gibbous moon, third quarter moon, and waning crescent moon.
• The Moon takes exactly as long to rotate on its axis as it does to make one orbit around Earth, which is called synchronous rotation and is the reason why we always see the same face of the Moon.
• A sidereal month is the time it takes the Moon to complete one full orbit of Earth as measured with respect to the stars, while the synodic month is the time it takes the Moon to complete one cycle as measured with respect to the Sun.

1-6 Eclipses occur only during rarely observed events when our Sun, Moon, and Earth are perfectly aligned

• An eclipse happens when the shadow of Earth falls on the Moon—a lunar eclipse—or when the Moon moves in front of the Sun and the shadow of the Moon falls on Earth—a solar eclipse.
• Eclipses can only happen when the Sun, Earth, and Moon align on the same ecliptic plane along a line called the line of nodes.
• In a lunar eclipse, the shadow of Earth has two distinct parts: the umbra, the darkest part of the shadow where no portion of the Moon’s surface can be seen, and the penumbra, which is not quite as dark and where a portion of the Moon’s surface is visible.
• If the Moon travels completely into the umbra, we have a total lunar eclipse; if only part of the Moon travels through the umbra, we see a partial lunar eclipse; if the Moon travels only through the penumbra, we see a penumbral eclipse.
• Lunar eclipses occur at full moon, and totality, the period when the Moon is completely within Earth’s umbra, can last for more than an hour.
• Because the angular diameter of the Moon is about the same as the larger but more distant Sun, the Moon just “fits” over the Sun during a total solar eclipse.
• During an eclipse, the Moon’s penumbra covers a large portion of Earth’s surface, and anyone inside the penumbra sees a partial solar eclipse.
• The eclipse path is widest if the Moon happens to be at perigee, the point in its orbit nearest Earth.
• If the Moon is at its farthest position from Earth, its apogee, the umbra may not reach Earth, resulting in an annular eclipse.