01 The Night Sky and Its Motions
Learn how Earth’s rotation and orbit shape the sky we see, and how the geometry of the Sun, Earth, and Moon explains seasons, lunar phases, and eclipses.
Mapping the sky
The is a useful way to map where objects appear in the sky. It is an imaginary sphere surrounding Earth; although stars lie at very different distances, their apparent directions can be plotted on its surface. The celestial equator is Earth’s equator projected onto the sky, and the celestial poles align with Earth’s rotation axis. The marks the Sun’s apparent yearly path across this sphere.
Like locations on Earth, positions in the sky can be described with coordinates. What an observer can see depends on their location: some parts of the sky remain below the horizon, while some stars circle a celestial pole without setting.
A is an officially defined region of the sky. The star patterns people recognize within those regions are useful landmarks, but the stars in a pattern may be far apart in space. Earth’s changing position in its orbit means different constellations are visible at night at different times of year.
Why the sky appears to move
Earth rotates from west to east, making the Sun, Moon, planets, and stars appear to move from east to west across the sky each day. This daily is mainly an effect of Earth’s rotation, not a daily sweep of the stars around Earth.
Stars near a celestial pole appear to circle it, while stars farther away rise and set across the horizon. Polaris, the North Star, lies close to the north celestial pole, so it appears nearly stationary to observers in the Northern Hemisphere.
Earth’s orbit around the Sun changes the direction into space that the nighttime side of Earth faces. As a result, the constellations visible in the evening shift with the seasons, and the Sun appears to move eastward against the background stars along the .
Takeaway: Earth’s rotation explains the sky’s daily ; Earth’s revolution around the Sun explains why the nighttime view changes over the year.
and the seasons
Earth’s is about relative to the plane of its orbit. As Earth revolves around the Sun, each hemisphere alternately tilts toward and away from the Sun. A hemisphere tilted toward the Sun receives more direct sunlight and has longer daylight hours, bringing warmer conditions. When tilted away, it receives less direct sunlight and has shorter days.
This is why seasons are caused primarily by Earth’s , not by Earth being substantially closer to the Sun in summer. The seasons are opposite in the Northern and Southern Hemispheres.
At a , a hemisphere is tilted most strongly toward or away from the Sun, marking its longest or shortest day of the year. At an , neither hemisphere tilts toward the Sun more than the other, and day and night are approximately equal in length.
Why the Moon changes phase
The Moon does not produce its own visible light: sunlight illuminates half of it. As the Moon orbits Earth, we see different portions of that sunlit half. These changing views are the , in order: new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter, and waning crescent.
Waxing means the visible lit portion is growing; waning means it is shrinking. The cycle from one new Moon to the next takes about days. The phases are not caused by Earth’s shadow. Earth’s shadow falls on the Moon only during a .
Eclipses and orbital alignment
An eclipse happens when the Sun, Earth, and Moon align closely enough for one body to cast a shadow on another.
A occurs at new Moon, when the Moon passes between Earth and the Sun and its shadow falls on part of Earth. Depending on the alignment and the bodies’ apparent sizes, it can be total, partial, or annular.
A occurs at full Moon, when Earth passes between the Sun and Moon and the Moon enters Earth’s shadow. It can be seen from the part of Earth experiencing nighttime where the Moon is visible.
Eclipses do not happen at every new and full Moon because the Moon’s orbit is tilted about relative to Earth’s orbital plane. Most months, the Moon passes a little above or below the alignment needed for an eclipse. An eclipse can occur when a new or full Moon happens near a point where the Moon’s orbit crosses that plane.
Takeaway: Moon phases follow the changing view of the Moon’s sunlit half; eclipses require a close alignment that also accounts for the tilt of the Moon’s orbit.