Questions and Exercises

Answers: 

2. Explain with the aid of fairly accurate diagrams, how the tilt of the earth’ s axis on its orbital plane around the sun causes: (a) the seasons (b) the variations in the length of day and night (c) the altitude of the midday sun to change at different times of the year?

Ans: The Earth's axial tilt, at approximately 23.5 degrees, is the primary reason for several phenomena we observe on Earth throughout the year. Let's explore how this tilt causes seasons, variations in day and night length, and changes in the altitude of the midday sun.

a. The Seasons

The tilt of the Earth's axis causes different parts of the Earth to receive more direct sunlight at different times of the year as the Earth orbits the sun. This variation in sunlight intensity and duration leads to the seasons.

• Summer: When the Northern Hemisphere is tilted towards the sun, it receives more direct sunlight for a longer duration. This increased solar radiation leads to warmer temperatures and summer in the Northern Hemisphere. Simultaneously, the Southern Hemisphere is tilted away from the sun, experiencing winter with less direct sunlight and shorter days.
• Winter: Conversely, when the Northern Hemisphere is tilted away from the sun, it receives less direct sunlight and shorter days, resulting in winter. At the same time, the Southern Hemisphere is tilted towards the sun, experiencing summer.
• Spring and Autumn (Equinoxes): During spring and autumn, neither hemisphere is tilted significantly towards or away from the sun. The sun's rays are most direct at the equator. This results in relatively equal day and night lengths across the globe and moderate temperatures, marking the transitional seasons of spring and autumn.

Diagrammatic Representation (Conceptual):

Imagine the sun at the center. As the Earth orbits around it, visualize the Earth's axis tilted at 23.5 degrees.

• June Solstice (Northern Hemisphere Summer): The Northern Hemisphere is tilted most directly towards the sun.
• December Solstice (Northern Hemisphere Winter): The Northern Hemisphere is tilted most directly away from the sun.
• March Equinox (Spring in Northern Hemisphere): Neither hemisphere is tilted towards or away from the sun.
• September Equinox (Autumn in Northern Hemisphere): Neither hemisphere is tilted towards or away from the sun.

b. Variations in the Length of Day and Night

The Earth's axial tilt also causes variations in the length of day and night throughout the year, except at the equator where day and night are always approximately 12 hours long.

• Summer Solstice (Longest Day): 
• What happens? One hemisphere (half of the Earth) is tilted most directly toward the Sun.
• Result:
• The Sun takes a longer path across the sky, so daylight lasts longer.
• Example: In the Northern Hemisphere, this happens around June 21. Days are very long (e.g., 16 hours of daylight), and places near the Arctic Circle have 24-hour sunlight!
• Meanwhile, the Southern Hemisphere experiences winter (short days).
• Winter Solstice (Shortest Day):

• What happens? One hemisphere is tilted farthest away from the Sun.

• Result:

• The Sun takes a shorter path across the sky, so daylight is brief.
• Example: In the Northern Hemisphere, this occurs around December 21. Days are very short (e.g., 8 hours of daylight), and the Arctic Circle has 24-hour darkness.
• Meanwhile, the Southern Hemisphere enjoys summer (long days).
• Equinoxes (Equal Day and Night): During the equinoxes (spring and autumn), the Earth's tilt is neither towards nor away from the sun. Consequently, day and night are approximately equal in length (about 12 hours each) at all latitudes.

Diagrammatic Representation (Conceptual):

Visualize the Earth rotating on its axis as it orbits the sun.

• Summer Solstice: For the summer hemisphere, more than half of its latitude circles are in sunlight during Earth's rotation.
• Winter Solstice: For the winter hemisphere, less than half of its latitude circles are in sunlight during Earth's rotation.
• Equinoxes: Exactly half of all latitude circles are in sunlight during Earth's rotation.

c. The Altitude of the Midday Sun

The altitude of the midday sun, which is the angle of the sun above the horizon at noon, changes throughout the year due to the Earth's tilt. This change is more pronounced as you move away from the equator towards the poles.

• Summer (Highest Altitude): In the summer hemisphere, because that hemisphere is tilted towards the sun, the sun appears higher in the sky at noon. This means the midday sun has a higher altitude angle, delivering more direct and intense sunlight.
• Winter (Lowest Altitude): In the winter hemisphere, the tilt away from the sun causes the sun to appear lower in the sky at noon. The midday sun has a lower altitude angle, resulting in less direct and less intense sunlight.
• Equinoxes (Intermediate Altitude): During the equinoxes, the midday sun's altitude is intermediate, as neither hemisphere is tilted significantly towards or away from the sun. The sun is directly overhead at the equator at noon during the equinoxes.

Diagrammatic Representation (Conceptual):

Imagine standing on Earth at a particular latitude.

• Summer: The sun's path across the sky is higher and longer, reaching a higher point at midday.
• Winter: The sun's path across the sky is lower and shorter, reaching a lower point at midday.
• Equinoxes: The sun's path is at an intermediate height, with the midday sun at a moderate altitude.

In summary, the Earth's axial tilt is the fundamental reason for seasons, variations in day and night length, and changes in the altitude of the midday sun throughout the year. These phenomena are interconnected and are all due to the Earth's orientation relative to the sun as it orbits.

3. Explain the differences between any three of the following: (a) perihelion and aphelion (b) parallels of latitude and meridians of longitude (c) the earth’ s rotation and the earth’ s revolution (d) solstice and equinox (e) Standard Time and Greenwich Mean Time

Answer: 

(c) Earth’s Rotation vs. Earth’s Revolution

Aspect	Rotation	Revolution
Definition	Earth spinning on its axis (imaginary line through poles).	Earth moving around the Sun in its orbit.
Time Taken	24 hours (causes day and night).	365.25 days (one year).
Effect	Creates the daily cycle of daylight and darkness.	Causes seasons (due to tilt + orbit).
Key Feature	Axis tilt (23.5°) does not change during rotation.	Orbit is elliptical (slightly oval-shaped).

Summary:

• Rotation = Spinning → Day/Night.
• Revolution = Orbiting → Seasons.

 

(d) Solstice vs. Equinox

Aspect	Solstice	Equinox
Occurrence	Twice a year (June 21 & December 21).	Twice a year (March 20 & September 22).
Sun’s Position	Sun directly over Tropic of Cancer (June) or Tropic of Capricorn (December).	Sun directly over the Equator.
Daylight	Longest day (summer solstice) or shortest day (winter solstice) in a hemisphere.	Equal day and night (~12 hours each).
Seasonal Impact	Marks the start of summer or winter.	Marks the start of spring or autumn.

Summary:

• Solstice = Extreme day length (longest/shortest).
• Equinox = Balanced day/night.

 

(e) Standard Time vs. Greenwich Mean Time (GMT)

Aspect	Standard Time	Greenwich Mean Time (GMT)
Definition	Official time of a region/country, based on longitude.	Time at the Prime Meridian (0° longitude, Greenwich, UK).
Purpose	Ensures uniform time within a time zone (e.g., Indian Standard Time).	Serves as the global reference for time zones.
Adjustment	Divided into 24 global time zones (each 15° longitude ≈ 1 hour).	Fixed to the solar time at Greenwich; no daylight saving.
Modern Use	Used locally (e.g., EST, IST, JST).	Replaced by UTC (Coordinated Universal Time) but still used informally.

Summary:

• Standard Time = Local time zone (e.g., your country’s clock).
• GMT = Global reference time (based on Prime Meridian).

 

4. Explain any three of the following terms connected with the earth and its planetary relations: i. galaxy ii. Pime Meridian iii. elliptical orbit iv. International Date Line

Answer:

i. Galaxy

A galaxy is a vast, gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies are the major building blocks of the universe.

• Vast Systems: Galaxies range in size from dwarf galaxies containing just a few billion stars to giant galaxies with trillions of stars.
• Gravity: All components of a galaxy are held together by the force of gravity.
• Types of Galaxies: Galaxies come in various shapes and sizes, broadly classified into spiral, elliptical, lenticular, and irregular galaxies.
• Earth's Galaxy: Earth is located in the Milky Way galaxy, a spiral galaxy. Our solar system is situated in one of the Milky Way's spiral arms, called the Orion Arm, about two-thirds of the way out from the galactic center.
• Scale: Galaxies are separated by immense distances and are constantly moving within the expanding universe.

ii. Prime Meridian

The Prime Meridian is the imaginary line of longitude on the Earth's surface defined to be at 0° longitude. It serves as the starting point for measuring longitude east and west around the Earth.

• Location: The Prime Meridian is conventionally defined as passing through the Royal Observatory in Greenwich, London, UK. This was internationally agreed upon in 1884.
• Longitude: Longitude lines run from the North Pole to the South Pole, and the Prime Meridian is the reference line from which all other longitude lines are measured, ranging from 0° at the Prime Meridian to 180° East and 180° West.
• Time Zones: The Prime Meridian is also the basis for Coordinated Universal Time (UTC), which is the primary time standard by which the world regulates clocks and time. Historically, Greenwich Mean Time (GMT) was based on the Prime Meridian at Greenwich.
• Navigation and Mapping: The Prime Meridian is fundamental for navigation, mapping, and global positioning systems, providing a consistent geographical reference.

iii. Elliptical Orbit

An elliptical orbit describes the path of a celestial body as it revolves around another celestial body in an ellipse, which is an oval shape. This contrasts with a perfect circle.

• Earth's Orbit: Earth, like all planets in our solar system, follows an elliptical orbit around the Sun.
• Focus Points: An ellipse has two focus points. In the case of planetary orbits, the body being orbited (like the Sun in our solar system) is located at one of these foci.
• Perihelion and Aphelion: Due to the elliptical nature of Earth's orbit:
• Perihelion: Earth is closest to the Sun at a point called perihelion, which occurs around January.
• Aphelion: Earth is farthest from the Sun at a point called aphelion, which occurs around July.
• Seasons (Note): While the elliptical orbit does cause slight variations in Earth's distance from the Sun, it is not the primary cause of seasons. Seasons are primarily caused by the Earth's axial tilt. The elliptical orbit does influence the length of seasons slightly, making summers in the Northern Hemisphere a bit longer and winters a bit shorter.

iv. International Date Line (IDL)

The International Date Line (IDL) is an imaginary line on the surface of the Earth, generally following the 180° longitude meridian. It marks the boundary where the date changes as you travel east or west across it.

• Location: The IDL runs roughly along the 180th meridian in the middle of the Pacific Ocean. It deviates in some places to avoid cutting through landmasses and political boundaries, mainly to keep countries and islands in the same date zone.
• Date Change:
• Westward Crossing: When you cross the IDL traveling westward (e.g., from America towards Asia), you advance one day. For example, if you cross it just after midnight on Tuesday, it becomes Wednesday.
• Eastward Crossing: When you cross the IDL traveling eastward (e.g., from Asia towards America), you go back one day. For example, if you cross it just after midnight on Wednesday, it becomes Tuesday.
• Purpose: The IDL is essential for maintaining consistent timekeeping and preventing confusion that would arise if each new time zone simply advanced the time without adjusting the date as one circumnavigates the globe.
• Practical Implications: For travelers crossing the IDL, it's important to adjust their calendars accordingly to stay synchronized with local time and data.

5. Give an explanatory account of the following. (a) Daylight increases as we go polewards in summer in the northern hemisphere. (b) The period of twilight in Britain is longer than in Malaysia. (c) A ship crossing the International Date Line at midnight on Wednesday east¬ wards finds that it is midnight, Tuesday, on the American side.

Answer:

a) Daylight increases as we go polewards in summer in the northern hemisphere.

This is due to the tilt of the Earth's axis and how it affects the angle at which sunlight reaches different latitudes, especially during summer in the Northern Hemisphere.

• Earth's Tilt: The Earth's axis is tilted at approximately 23.5 degrees relative to its orbital plane. During the Northern Hemisphere's summer (around June solstice), the North Pole is tilted towards the Sun.
• Sunlight Angle and Day Length: Because of this tilt, the Northern Hemisphere receives more direct sunlight. As you move from the equator towards the North Pole in the summer:
  • Equator: At the equator, daylight hours are consistently around 12 hours throughout the year.
  • Mid-latitudes: In mid-latitudes (like those of Europe and the USA), summer days are longer than winter days, but the difference is noticeable.
  • Polar Regions: As you approach the Arctic Circle (66.5°N latitude) and beyond:
    • During summer, the tilt causes the sun to remain above the horizon for more than 12 hours each day.
    • At the Arctic Circle, there is one day of 24-hour daylight (Summer Solstice).
    • Further poleward, the period of continuous daylight increases. For example, at 80°N latitude, there are several weeks of continuous daylight.
    • At the North Pole itself, there are about 6 months of continuous daylight during summer.
• Reason: The tilt causes the polar regions to be within the sun's illumination for a greater portion of the Earth's rotation during their respective summers. Imagine the Earth as tilted 'leaning into' the sun in the summer hemisphere. This 'lean' means that as the Earth rotates, locations closer to the pole in the summer hemisphere spend more time in the sunlight.

(b) The period of twilight in Britain is longer than in Malaysia.

Twilight is the period after sunset and before sunrise when the sky is partially illuminated. The duration of twilight depends on latitude.

• Twilight Definition: Twilight occurs because the Earth's atmosphere scatters sunlight even when the sun is below the horizon.
• Latitude and Twilight Angle:
  • Equatorial Regions (like Malaysia): Malaysia is close to the equator. Near the equator, the sun's path is almost perpendicular to the horizon. The sun sets and rises more vertically. This means the sun moves through the twilight zone (between the horizon and 18 degrees below the horizon, which defines civil twilight) relatively quickly. Therefore, twilight is shorter.
  • Higher Latitudes (like Britain): Britain is at a higher latitude. At higher latitudes, the sun's path is at a shallower angle to the horizon, especially during summer. The sun sets and rises at a more oblique angle. This means the sun takes longer to move through the twilight zone (18 degrees below the horizon). Consequently, twilight lasts longer.
• Summer Effect: The effect is more pronounced in summer at higher latitudes because the sun's overall path across the sky is shallower and longer, leading to extended periods where the sun is just below the horizon before complete darkness sets in, and similarly after sunrise.

(c) A ship crossing the International Date Line at midnight on Wednesday eastwards finds that it is midnight, Tuesday, on the American side.

The International Date Line (IDL) is designed to manage the change of date as you travel around the world and cross time zones.

• Purpose of the IDL: As you travel eastward around the world, you are effectively moving forward in time. If you simply kept advancing your clock by one hour for each time zone, you would gain 24 hours after circling the globe. Conversely, traveling westward would result in losing 24 hours. To correct this, the IDL was established.
• Eastward Crossing - Date Loss: When you cross the IDL going eastward, you move to an earlier date.
  • Scenario: If a ship is crossing the IDL at midnight on Wednesday while traveling eastwards, it is moving from a time zone where it is Wednesday into a time zone immediately to its east where it is still Tuesday.
  • Date Adjustment: To account for this, upon crossing the IDL eastward, you must subtract a day. So, midnight Wednesday immediately becomes midnight Tuesday.
• Westward Crossing - Date Gain: Conversely, if you cross the IDL going westward, you move to a later date. You would add a day.
• Midnight Example: In your example:
  • On the Asian side of the IDL, it's midnight Wednesday.
  • As the ship crosses eastward to the American side, it instantly becomes midnight Tuesday. This is because locations to the east of the IDL are always a day behind locations to the west.

In summary, these phenomena are all consequences of Earth's spherical shape, its axial tilt, and the system of time zones and date lines humans have created to navigate and understand time and seasons on our planet.