Chapter 15 – Earth and space

Consider the following statements and click to reveal the answer.

1. Which force will determine the ultimate fate of the Universe?


Gravity, one of the most fundamental forces of nature. Gravity is responsible for forming galaxies and stars and probably planets throughout the Universe even today. Gravity also keeps our feet on the ground wherever we stand on the Earth.

2. Which observational feature of galaxies do you think first suggested that they might be rotating?


The arms of spiral and barred spiral galaxies. Their appearance is similar to that produced by a spinning Catherine wheel. Astronomers have confirmed galactic rotation using more advanced scientific techniques.

3. Stars are born, they live and then they die. Using the knowledge and understanding of life processes gained from earlier chapters, what do you think are the advantages and disadvantages of this ‘life cycle’ analogy?


Most analogies are productive in that they use comparisons with more familiar objects and events to make difficult concepts and processes easier to understand. Stars, of course, are not like humans and other animals or plants as they are not alive.

4. If the Sun is the only object within the Solar System to produce its own light, how are we able to see the planets, their moons and the other objects around?


We can see planets, moons and other objects in the Solar System because they reflect light from the Sun. 

5. Why is it dangerous to look at the Sun without proper eye protection?


If you look directly at the Sun, the eye tissues transmit a substantial part of the solar radiation to the retina and the intense visible light causes damage to its light-sensitive rod and cone cells. The light sets off a series of complex chemical reactions which damage the cells’ ability to respond to a visual stimulus and, in extreme cases, can destroy them. The result is a loss of vision which may be either temporary or permanent, depending on the severity of the damage. If you look repeatedly or for a long time at the Sun without proper eye protection, you may also suffer a thermal injury: the high level of radiation causes heating that literally cooks the exposed tissue. This also will destroy the rod and cone cells, creating a small blind area. This is a significant danger, as such injuries do not cause any feeling of pain (there are no pain receptors in the retina), and you would not experience the effects of the damage on your sight for at least several hours after it is done. (For further information, see ‘Eye safety during solar eclipses’, available at:

6. Why do you think that ancient astronomers believed in a Universe with the Earth at its centre?


The Sun, the Moon, stars and visible planets all appear to ‘rise’ in the east and ‘set’ in the west. The most obvious conclusion to draw was that a motionless Earth sat at the centre of all astronomical activity. As detailed observational evidence and other ideas emerged over the millennia that followed, particularly during the sixteenth and seventeenth centuries with the work of Copernicus and Galileo, this view was eventually challenged.

7. List the six major components of the Solar System.


The Sun, the planets, moons, asteroids, comets and meteoroids are the major components of the Solar System.

8. On any given day of the year, who would see the Sun ‘rise’ first: people living in New York or London?


As the Earth rotates from west to east, people living in London would see the Sun rise first.

9. How long is a year and how do you think that it is measured?


A year is 365.25 days long. This is the time taken for the Earth to complete one orbit of the Sun. But how is that known? If you could observe where the Sun appears on the horizon on any given day of the year, it would take 365.25 days for the Sun to return to that same position again, having moved once through all of the variations described.

10. How long is a twelve-month lunar year in days and why do you think that lunar years are not the best timekeepers?


A lunar year is 12 × 29 days 12 hours long. A 354-day lunar year is 11.25 days shorter than the time taken for the Earth to orbit the Sun. Keeping time with the Moon results in problems associated with fixing the time of the seasons. Certain religious festivals, including Ramadan and Easter, are linked to lunar cycles. This is why they vary in time every year.

11. New moons and full moons occur within 29-day, 12-hour lunar cycles but eclipses only happen once or twice per year. Why do eclipses not occur more often?


This is because the plane of the Moon’s orbit around the Earth is tilted at an angle of about 5° relative to the plane of the Earth’s orbit around the Sun. Most of the time the Sun, the Earth and the Moon are not perfectly aligned.

12. Sketch how the Earth–Sun–Moon System would look in June. Which hemisphere is experiencing summer and why? Describe the Sun’s location and apparent movement in the sky from ‘sunrise’ to ‘sunset’ as viewed from the UK. Assume that the Moon is in its full moon phase. Where would it appear in your sketch? When would it be visible from the UK?


In June, the northern hemisphere is tilted towards the Sun and experiences summer. Days are long, the Sun’s rays reach the surface of the Earth at high angles and the Sun’s heating effect is more efficient. The Sun ‘rises’ in the northeast, ‘passes’ at its highest in the sky and ‘sets’ in the northwest. At full moon, the Moon would only be visible at night. Check its location against text and chapter illustrations.