knowverseWhat causes the seasons? Earth's axial tilt of about 23.5 degrees, which changes the angle of sunlight and the length of daylight through the year. The quiz covers the ecliptic, the equinox, moon phases, solar and lunar eclipses, and why eclipses do not happen every month. Try the ten questions now.
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The ecliptic is the apparent path traced by the Sun through the constellations over one full orbit of Earth.
The imaginary line that the Sun traces against these background stars over the course of one full year is called the Ecliptic.
From the Knowverse course text.
Earth's axial tilt changes sunlight angles and daylight duration in each hemisphere through the year.
Earth does not sit perfectly upright as it travels through space; instead, it leans at an angle of about 23.5 degrees.
From the Knowverse course text.
An equinox is the instant the Sun crosses the celestial equator. Around this date, day and night are approximately equal in length.
An Equinox occurs at the exact moment the Sun crosses the celestial equator.
From the Knowverse course text.
Outside eclipses, roughly half the Moon is sunlit. As it orbits Earth, our view of that illuminated half changes.
The reason the Moon seems to change shape is that we see different amounts of that lit-up half as the Moon orbits the Earth.
From the Knowverse course text.
A crescent moon occurs when only a thin curved fraction of the sunlit hemisphere faces Earth.
A Crescent moon is a thin, curved sliver of light that looks like a fingernail or a banana.
From the Knowverse course text.
Earth moves around the Sun during the Moon's orbit. The Moon needs about 2.2 additional days to regain the same Sun-relative phase alignment.
While the Moon spent 27.3 days circling us, Earth was also traveling along its own path around the Sun.
From the Knowverse course text.
The umbra is the darkest central cone of the shadow where the Sun is completely blocked by the Moon.
This shadow is not the same everywhere; it has a very dark center called the Umbra.
From the Knowverse course text.
Rayleigh scattering removes shorter blue wavelengths, while Earth's atmosphere bends the remaining red light onto the Moon.
However, red light passes through the atmosphere much more easily. During an eclipse, the Earth's atmosphere bends this red light inward, aiming it directly at the Moon.
From the Knowverse course text.
Nodes are the intersections where the Moon's five-degree tilted orbital path crosses the flat ecliptic plane.
The two specific points where the Moon's tilted orbit crosses the flat plane of Earth's orbit are called nodes.
From the Knowverse course text.
Because of the five-degree tilt, the Moon usually passes above or below Earth's orbital plane, missing eclipse alignment.
The Moon's orbit is tilted about 5 degrees relative to Earth's orbit around the Sun. Because of this tilt, the Moon usually passes slightly above or below the Sun from our perspective.
From the Knowverse course text.
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The ecliptic is the Sun's apparent yearly path against the background stars. Earth's axis is tilted by about 23.5 degrees relative to the perpendicular to its orbital plane. Over a year, this makes the hemispheres take turns receiving more direct sunlight, which concentrates incoming energy onto a smaller area.
The axis points in approximately the same direction during a year. When the Northern Hemisphere leans toward the Sun, the Southern Hemisphere leans away. Solstices mark the Sun's greatest northern or southern declination. Equinoxes occur when it crosses the celestial equator, with day and night roughly equal in length.
The imaginary line that the Sun traces against these background stars over the course of one full year is called the Ecliptic.
From the Knowverse course text.
Moon phases show changing views of the Moon's sunlit half as it orbits Earth. A crescent shows a thin illuminated sliver. A cycle of phases takes about 29.5 days, longer than the roughly 27.3-day orbit measured against the stars because Earth also moves around the Sun during that time.
Eclipses require a closer alignment. In a solar eclipse, the Moon blocks the Sun from part of Earth. In a lunar eclipse, the Moon enters Earth's shadow. Its orbit is tilted about five degrees to Earth's orbital plane, so most new and full moons miss the required alignment near the orbital crossing points, called nodes.
The reason the Moon seems to change shape is that we see different amounts of that lit-up half as the Moon orbits the Earth.
From the Knowverse course text.
Sources: Earth and Moon Dynamics. Each question or step includes an excerpt from the course.
