Lecture 2: Observing the Sky, Lecture notes of Astronomy

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Observing

the Sky

LECTURE 2

I. BASICS

ZENITH

Point on the celestial sphere directly above the observer’s position on Earth (point of where you are). Point over your head. If looking up into the sky, the zenith is the point where your line of sight intersects with the celestial sphere.

HORIZON

A flat tangent to the Earth’s surface where the observer is standing (line that connects the sky to the ground/sea). Appears as a straight line in human perspective.

NADIR

Opposite of zenith. Point directly below the observer’s position on Earth (directly beneath your feet). Lowest point in the sky.

CELESTIAL SPHERE

Giant, imaginary spherical dome that surrounds the Earth. Rotates as Earth does on its axis. A reference point for measuring and calculating positions of celestial objects.

I. BASICS “Circumpolar Stars” => “Never rise, never set” => Stars that never rise or set due to it’s proximity with the celestial poles and observer’s latitude => Stars at the equator are opposite, none are circumpolar). => Circumpolar stars are always visible above the horizon from said location every night. => Spins in a circular path around the celestial poles without disappearing below the horizon due to Earth’s rotation. => Useful for determining time or directions. Height of Polaris will tell your latitude. => AKA North Star, located above the North Pole (Stays in the same position in the sky while other stars rotates around it). => Useful as a reference point for navigation since the height of Polaris above the horizon is approximately the same value as your latitude. => By measuring the angle between the Earth’s horizon and Polaris, you can determine latitude (higher Polaris = the farther north you are).

II. EARTHY CYCLES:

Sidereal Day:

Based on the time it takes for Earth to complete one full rotational period relative to distant stars and not to the Sun. Approximately 23hours, 56 minutes and 4 seconds long. Shorter than solar day.

Solar Day:

True rotational period Earth about its axis. Approximately 24 hours long. Facing the Sun = Noon; Facing away from the Sun = Midnight. Facing the Sun = Noon Facing away from the Sun = Midnight DAY & NIGHT TIDES Rise and fall of sea level caused by the gravitational pull of the Moon and the Sun on Earth’s ocean. High tide = when the Moon’s gravitational pull on Earth’s ocean. Low tide = when waves of water experience low water levels. Sun’s effect on the ocean is weaker than the Moon. When Sun, Moon and Earth are aligned their gravitational pull creates a higher high tides and lower low tides (often occurs during full moons or new moons). High tide and low tide occurs every 6 hours. New and Full Moons = most pronounced tides. Earthquakes are most likely to occur during Full or New Moon due to the pull ofthe tides. Earth can also cause tides on the Moon. As Earth rotates, it takes the tides with itself, causing friction as the tides want to face the Moon.

ECLIPSES Solar Eclipse Occurs when the Moon blocks the Sun’s light, casting it’s shadow on Earth. Occurs when Moon passes directly through the Earth’s shadow. the Moon gets darker. Total Lunar Eclipse : Earth blocks the Sun’s light from reaching the Moon, causing reddish color. Partial Lunar Eclipse : Some part of Moon get’s block by Earth’s shadow,. Penumbral Lunar Eclipse : Outer part of the Moon passes through Earth’s shadow, causing a dimmer look than normal. Lunar Eclipse (^) Moon phases do not change. Sun gets darker. Shorter than lunar eclipse. II. EARTHY CYCLES: Occurs when Moon passes directly through the Earth’s shadow. Appears dark or reddish. Opposite of the sun. Occurs every 6 months during the full moon phase. Happens during the day where daylight becomes dark. Lasts for a few minutes in only few places every year. Total Solar Eclipse : Moon completely covers the Sun, causing it to be night time during daylight hours. Partial Solar Eclipse : Some part of the Sun get’s block by the Moon. Annular Solar Eclipse : Moon is too far from Earth to completely block the Sun. Only the rings of the Sun is seen.