Origin of Sunlight Energy (light) is produced by hydrogen fusion at the center of the Sun (see Lecture 11 for more details). These photons of light do not leave the Sun immediately, and instead bounce from atom to atom until finally escaping into space. The journey of a photon from the center to the surface of the Sun takes 10 million years!
The Sun’s Differential Rotation The Sun does not rotate as a solid body: its equator rotates once every 25 days, while regions near the poles rotate every 30 days.
The Sun’s Magnetic Field Imagine the Sun as a bar magnet, with magnetic field lines cutting through it. The field lines are attached to the Sun. After a while, differential rotation stretches and stresses the field lines. Kinks develop.
Stretching the Magnetic Field The magnetic field kinks appear on the surface as pairs of sunspots. The spots appear dark because they are cooler than their surroundings – their energy is stored in the magnetic field.
Prominences and Flares Eventually, something has to give. Just like a rubber band, the field lines will break and release their energy. Solar Flare Solar Prominence
The Sun in X-rays Because the Sun’s temperature is about 6000°, it emits mostly at optical wavelengths. However, solar flares are extremely energetic explosions – they emit in the x-ray part of the spectrum.
The Solar Cycle Once the magnetic field lines reconnect, the cycle begins again. Observations show that it takes about 11 years before the lines get stretched to the breaking point.
The Solar Cycle Once the magnetic field lines reconnect, the cycle begins again. Observations show that it takes about 11 years before the lines get stretched to the breaking point. Solar Maximum March 29, 2001 Solar Minimum Oct. 11, 2004
The Maunder Minimum Sunspots are easy to spot –you don’t need a telescope (just project the sun through a pinhole). So good data on the Sun exists all the way to the time of Galileo. In the 1600’s, the earth went through a mini-ice age: Europe and Asia were abnormally cold. Apparently the solar cycle hasn’t always been as regular as it has been recently.
The Sun and Earth When the Sun has a lot of sunspots, solar flares, and prominences • The earth is warmed by all the additional energy • The earth is bombarded with cosmic rays (i.e., high energy hydrogen and helium nuclei that are ejected from the Sun. In other words, a stronger solar wind.) The earth’s magnetic field and atmosphere protects us from these particles; those that get through are funneled into the atmosphere at the poles.
Aurorae When the solar wind hits the earth’s atmosphere, the particles excite electrons bound to atoms of oxygen and nitrogen. When the electrons fall back down, they produce emission lines.
Tides on Earth A tidal force is the difference in gravity from one side of a body to the other that is exerted by a 2nd object. The Moon exerts a tidal force on the Earth that causes the oceans facing the Moon to bulge out toward it, and the oceans on the opposite side of the Earth to bulge out away from the Moon. These varying ocean levels are called the tides.
Tides on Earth The Sun also causes tides on the Earth, but they are smaller than the Lunar tides. When the Moon and Sun are aligned, their tidal forces combine, and the resulting tides are largest.
Tidal Friction The Earth is constantly rotating, pulling the tidal ocean bulge out of alignment. As a result, the water is continually moving in the opposite direction of the Earth’s rotation.
Tidal Friction The movement of the water on Earth has two effects: • It slows down the Earth’s rotation (few milliseconds per century). When dinosaurs roamed the Earth, a day was 22 hours long. Eventually, the Earth will rotate once in the same time the Moon orbits once. So the Earth and Moon will always have the same faces toward each other.
Tidal Friction The movement of the water on Earth has two effects: • It pulls the Moon along a bit faster, slinging it out further from the Earth (about 4 cm per year). So in the distant past, the Moon was much closer to the Earth.
Tides on Moon The tidal forces of the Moon on the Earth are slowing the Earth’s rotation so that it will keep the same face toward the Moon. Since the Earth is about 80 times more massive than the Moon, its tidal force is 80 times greater. So tidal friction of flowing rocks (lava) has already locked the Moon to the Earth. This is why the Moon always keeps the same side facing the Earth. From the Earth, we never see the far side of the Moon.
Formation Scenarios for the Moon Four major theories have been proposed for formation of the Moon: Fission: the Moon broke off of the Earth Co-formation: Moon formed like the Earth, right next to the Earth Capture: Moon formed elsewhere in the solar system and was later captured by Earth’s gravity Large impact: Mars-size planet collided with the Earth and the Moon formed from the debris The Moon has a similar composition as the Earth’s crust and mantle, but has a much smaller iron core. If the Moon formed by fission or co-formation, it should have a larger iron core like the Earth. If it formed through capture, it shouldn’t match the composition of the Earth’s crust and mantle.
Formation Scenarios for the Moon The large impact theory is widely believed to be correct. The iron core of the impacting planet could have merged with the Earth’s core, while the Moon formed from crust and mantle thrown into space. This explains why the Moon in similar in composition to the Earth’s crust and mantle, but has as very small iron core. The impact theory also explains why rocks on the Moon contain no water or other volatiles (easily evaporated materials).
Effects of the Moon on the Earth This impact that created the Moon may have caused the Earth’s spin axis to become tilted. So we might not have seasons if it wasn’t for this collision. The Moon has several effects on the Earth that are probably beneficial for life. Without the Moon: • There would be no lunar tides in the ocean. Only the much smaller tides from the Sun would remain. Tides may have helped wash minerals into the ocean that were needed for the formation of life. • The Earth’s spin would not have slowed down, and the day would be as short as when the Earth was born (6 hours). With such fast rotation, the atmosphere would have much stronger winds, producing stronger ocean waves. Early organisms would have taken more of pounding. • The tilt of the Earth’s axis would have been unstable, and could have changed drastically from time to time, which would have produced huge climate changes and earthquakes.
When the Moon formed, it was about 10 times closer to the Earth than it is now. At this distance, the tidal forces would have been 1000 times stronger, resulting in ocean tides that rushed miles inland and out to sea every day (which was only 6 hours long).