Astronomy Honor

Nature Study

Requirements

1. Answer the following questions:
   • What causes an eclipse?
   • What is a shooting star?
   • What is the speed of light?

   Answer: 1) What causes an eclipse? — An eclipse occurs when a celestial body blocks the light of another by casting its shadow. In a solar eclipse, the Moon passes between the Sun and the Earth (in the New Moon phase) and casts its shadow on the Earth, darkening the Sun; it is seen in a narrow band and lasts a few minutes (it can be total, partial, or annular). In a lunar eclipse, the Earth is between the Sun and the Moon (in the Full Moon phase) and the Earth's shadow covers the Moon, which darkens and turns reddish ('Blood Moon'); it is seen across the entire night hemisphere and lasts longer. Eclipses do not happen every month because the Moon's orbit is tilted about 5° relative to the Earth's, so that perfect alignment (at the points called nodes) is rarer. 2) What is a shooting star? — It is a phenomenon called a meteor: it is not actually a star, but a small fragment of rock or dust from space (a meteoroid) that enters the Earth's atmosphere at high speed and, through friction with the air, heats up and burns, producing a luminous trail in the sky. When the fragment does not burn up completely and reaches the ground, it is called a meteorite. 3) What is the speed of light? — Approximately 300,000 km per second (more precisely 299,792 km/s) in a vacuum. — The speed of light is so extreme that, in 1 second, light circles the Earth 7.5 times — a principle that limits communications with space probes (signals to Mars take 4-24 minutes, depending on the orbital position).

2. About the celestial body closest to Earth, answer:
   • Give its name.
   • What is its distance from Earth?
   • What is its influence on the tides?

   Answer: 1) Give its name — the Moon, Earth's only natural satellite and the celestial body closest to it. 2) What is its distance from Earth? — On average, about 384,400 km, varying between approximately 363,000 km (perigee, when it is closest) and 405,000 km (apogee, when it is farthest). 3) What is its influence on the tides? — The Moon's gravitational force attracts the ocean waters, raising the sea level on the side of the Earth facing it (and also on the opposite side), which causes high tides; in the intermediate regions low tides occur. That is why there are, in general, two high tides and two low tides per day. The Sun also has an influence, but with a smaller effect; when the Sun, Earth and Moon align (new and full Moon) the tides become more intense (spring tides), and in the quarter phases they become weaker (neap tides). — The Moon moves away from the Earth about 3.8 cm per year, due to the transfer of angular momentum through the tides — a phenomenon discovered by the LLR (Lunar Laser Ranging) program using reflectors left by the Apollo 11 mission since 1969.

3. Make a diagram showing the relative positions and movements of the Earth, Sun and Moon. Show the positions and movements in lunar and solar eclipses.

   Answer: Basic diagram: the Sun at the center; the Earth orbiting the Sun in ~365 days (24h rotation, axis tilted 23.5°); the Moon orbiting the Earth in ~27.3 days. SOLAR eclipse — Sun-Moon-Earth alignment: the Moon passes between the Sun and the Earth, its shadow reaches the Earth (umbra = total eclipse. — The 'blood moon' (total lunar eclipse) has a reddish color because the Earth's atmosphere filters the sunlight, letting only the red wavelengths pass through — the same principle as a sunset, projected onto the Moon.

4. Make a diagram of the solar system, naming the planets in the correct order from the Sun.

   Answer: Order of the 8 planets from the Sun: (1) Mercury, (2) Venus, (3) Earth, (4) Mars, (5) Jupiter, (6) Saturn, (7) Uranus, (8) Neptune. The first 4 are rocky (terrestrial) planets; the last 4 are gas giants. — Jupiter is so large (mass 318× greater than the Earth) that it harbors 95 known moons and has stormy spots (the 'Great Red Spot') larger than the entire Earth itself; it is the largest planet in the solar system, with a diameter of about 140,000 km.

5. Identify 5 fixed stars in the sky. What is the difference between a planet and a fixed star?

   Answer: Five fixed stars that are easy to identify: Sirius (the brightest in the sky, in the Canis Major constellation), Canopus (the second brightest, in the southern hemisphere), Alpha Centauri (the closest system, ~4.3 light-years), Vega (in Lyra), and Aldebaran/Antares (red, in Taurus and Scorpius) — or Rigel and Betelgeuse, in Orion. The difference between a planet and a fixed star: a fixed star is a distant sun that produces its own light (it shines through nuclear reactions), is light-years away, and therefore appears to 'blink/twinkle' and always keeps the same position relative to the other stars. A planet has no light of its own — it only reflects the light of the Sun —, is much closer (within the Solar System), shines with a steady light (does not twinkle), and changes position in the sky night after night, 'wandering' among the stars (hence the Greek word planet = wanderer). — The twinkling of the stars occurs because the light, coming from extremely distant points, crosses layers of air with different temperatures; planets have a larger image (despite less apparent brightness) and therefore twinkle less.

6. About constellations:
   • Explain what a constellation is.
   • Name and point out 6 different constellations.
   • Name 2 constellations that are visible throughout the year.

   Answer: A constellation is an apparent grouping of stars that form imaginary figures in the sky. There are 88 official constellations recognized by the International Astronomical Union (IAU) since 1930. Six examples: (1) the Southern Cross (Crux); (2) Orion (the hunter); (3) Scorpius; (4) Taurus; (5) Canis Major; and (6) Ursa Major. Each season of the year reveals different constellations in the sky. — The Southern Cross appears on the flags of Brazil, Australia, New Zealand, Samoa, and Papua New Guinea — all southern hemisphere nations whose historical navigation depended on this constellation to locate the south celestial pole before modern GPS.

7. Draw a diagram of the Southern Cross, Orion and Scorpius constellations.

   Answer: SOUTHERN CROSS (Crux): 4 main stars forming a cross — Acrux (α, the base, brightest), Becrux (β, left side), Gacrux (γ, top), Decrux (δ, right side), and Epsilon Crucis (ε, the smallest). — Betelgeuse (Orion) is a red supergiant 1,000× larger than the Sun — astronomically close to exploding as a supernova; when it explodes (in the next 100,000 years), it will shine like the full moon even during the day across the entire planet Earth.

8. What is the Milky Way? Observe the Milky Way in the night sky.

   Answer: The Milky Way is the spiral galaxy in which our Solar System is located. It has about 100-400 billion stars, a diameter of approximately 100,000 light-years and a thickness of 1,000 light-years. Our Sun is in the Orion arm, about 26,000 light-years from the galactic center. — The name 'Milky Way' comes from the Latin 'via lactea' (road of milk) — the ancient Greeks believed it was milk spilled by the goddess Hera; the whitish appearance comes from the diffuse glow of billions of stars too distant to be seen individually.

9. What is the morning star or evening star? Why does it receive these names?

   Answer: It is the planet VENUS, the third brightest object in the sky (after the Sun and the Moon). It receives these two names because, since Venus orbits the Sun closer than the Earth, it never moves far from the Sun in the sky and is only visible shortly before dawn or just after dusk, never in the middle of the night. It is called the 'morning star' (Phosphorus/Lucifer/Daystar) when it appears in the east, in the sky before sunrise; and the 'evening star' (Hesperus/Vesper) when it appears in the west, just after sunset. Despite the name, it is not a star — it is a planet that merely reflects the light of the Sun. — Venus can be so bright that it casts shadows on the Earth on dark, moonless nights — a rare phenomenon confirmed in long-exposure photographs; its luminosity comes from the thick layer of acidic clouds that reflects 75% of the sunlight.

10. Explain what the zenith and nadir are.

    Answer: Zenith: an imaginary point directly above the observer's head (90° above the horizon, perpendicular to the ground). Nadir: the exactly opposite point, below the observer's feet (passing through the Earth). Together they form the vertical axis of the celestial sphere. — At the equator, on dates close to the equinoxes (March 21 and September 22), the Sun is exactly at the zenith at noon, creating the phenomenon where vertical objects cast no shadow at all — an impressive effect demonstrable in any equatorial city.