(A) The age of the Universe is estimated to be 13.8 billion years. How was it estimated? (B) Explain the phenomenon of Solar Eclipse. (C) Describe the James Webb Space Telescope, its principles of operation, construction and importance. (D) Explain the Global Positioning System and its applications.

(A) How the age of 13.8 billion years was estimated

Cosmic microwave background (CMB): this is the main method. The CMB is faint radiation released when the Universe first became transparent. Tiny patterns in it depend on the Universe's composition and expansion. By fitting these patterns to the standard cosmological model, the Planck satellite obtained an age of about 13.8 billion years.

Expansion rate: by Hubble's law, galaxies recede at speeds proportional to their distances. Tracing the expansion backward gives a rough age of 1/H₀, where H₀ is the Hubble constant. This is then corrected for changes in the expansion rate caused by matter and dark energy.

Oldest objects: as an independent check, the Universe must be older than its oldest stars. The ages of globular star clusters and white dwarf cooling suggest ages of about 12 to 13 billion years or more, consistent with this figure.

(B) Solar eclipse

A solar eclipse occurs when the Moon passes directly between the Sun and the Earth, blocking the Sun's light from part of the Earth. It occurs only at new moon, and not every month, because the Moon's orbit is tilted about 5° to the Earth's orbit.

It is possible because the Sun is about 400 times wider than the Moon, but also about 400 times farther away, so they appear about the same size in the sky.

Total eclipse: seen within the Moon's dark shadow (umbra), where the Sun is entirely covered and its corona is visible. Partial eclipse: seen within the lighter shadow (penumbra), where only part of the Sun is covered. Annular eclipse: occurs when the Moon is farther from the Earth and appears slightly smaller, leaving a bright ring of sunlight.

The total phase is only visible along a narrow path and lasts a few minutes at most. The Sun should never be viewed without proper eye protection.

(C) James Webb Space Telescope

The James Webb Space Telescope (JWST) is a large infrared space observatory, developed by NASA, the European Space Agency and the Canadian Space Agency, and launched on 25 December 2021.

Principles of operation: its main mirror collects faint infrared light and reflects it to a secondary mirror, which directs it to sensitive instruments. Infrared light is especially valuable, as light from very distant galaxies is stretched to longer wavelengths by cosmic expansion (redshift), and infrared can also pass through dust clouds. Since warm objects emit infrared, the telescope must remain extremely cold.

Construction: its 6.5-metre primary mirror has 18 gold-coated beryllium segments, far larger than the Hubble Space Telescope's 2.4-metre mirror. A tennis-court-sized, five-layer sunshield blocks heat from the Sun, Earth and Moon. Its instruments include NIRCam, NIRSpec, MIRI and NIRISS. It orbits around the L2 point, about 1.5 million km from Earth.

Importance: it observes some of the first galaxies, studies how stars and planetary systems form, analyses the atmospheres of exoplanets for molecules such as water and CO₂, and provides information on the early Universe and galaxy evolution.

(D) Global Positioning System

The Global Positioning System (GPS) is a US satellite navigation system that gives position and time anywhere on Earth. It uses at least 24 satellites at about 20,200 km altitude, along with ground stations and user receivers. Each satellite broadcasts its position and precise time. A receiver measures signal travel times to find its distance from at least four satellites, and it then calculates its position by trilateration, with the fourth signal correcting its clock error.

Applications: navigation for vehicles, ships and aircraft; mapping and surveying; precision farming; emergency response and disaster relief; fleet tracking and delivery; military operations; location services on phones; and accurate timing for banks and telecommunications networks.