Book contents
- Frontmatter
- Dedication
- Contents
- Preface
- Acknowledgements
- 1 Watchers of the skies
- 2 Our Sun
- 3 Aspects of our Solar System
- 4 The rocky planets
- 5 The hunt for Planet X
- 6 Voyages to the outer planets
- 7 Harbingers of doom
- 8 Impact!
- 9 Four hundred years of the telescope
- 10 The family of stars
- 11 Aging stars
- 12 The search for other worlds
- 13 Are we alone? The search for life beyond the Earth
- 14 Our island Universe
- 15 Wonders of the southern sky
- 16 Proving Einstein right
- 17 Black holes: no need to be afraid
- 18 It’s about time
- 19 Hubble’s heritage: the astronomer and the telescope that honours his name
- 20 The violent Universe
- 21 The invisible Universe: dark matter and dark energy
- 22 The afterglow of creation
- 23 To infinity and beyond: a view of the cosmos
- Index
- Plate section
- References
18 - It’s about time
Published online by Cambridge University Press: 05 October 2014
- Frontmatter
- Dedication
- Contents
- Preface
- Acknowledgements
- 1 Watchers of the skies
- 2 Our Sun
- 3 Aspects of our Solar System
- 4 The rocky planets
- 5 The hunt for Planet X
- 6 Voyages to the outer planets
- 7 Harbingers of doom
- 8 Impact!
- 9 Four hundred years of the telescope
- 10 The family of stars
- 11 Aging stars
- 12 The search for other worlds
- 13 Are we alone? The search for life beyond the Earth
- 14 Our island Universe
- 15 Wonders of the southern sky
- 16 Proving Einstein right
- 17 Black holes: no need to be afraid
- 18 It’s about time
- 19 Hubble’s heritage: the astronomer and the telescope that honours his name
- 20 The violent Universe
- 21 The invisible Universe: dark matter and dark energy
- 22 The afterglow of creation
- 23 To infinity and beyond: a view of the cosmos
- Index
- Plate section
- References
Summary
As the title of this chapter implies, it is about the enigmatic concept of time – perhaps the least understood topic in physics. It is almost impossible to give a definition of time, and perhaps the most understandable was provided by the physicist John Wheeler, whose definition was ‘Time is nature’s way of preventing everything happening at once’!
Let us first discuss how astronomers measured the passage of time until the 1960s.
Local Solar Time
For centuries, the time of day was directly linked to the Sun’s passage across the sky, with 24 hours being the time between one transit of the Sun across the meridian (the line across the sky from north to south) and that on the following day. This time standard is called ‘Local Solar Time’ and is the time indicated on a sundial. The time such clocks showed would thus vary across the UK, as noon is later in the west. It is surprising the difference this makes. In total, the UK stretches 9.55 degrees in longitude from Lowestoft in the east to Manger Beg in County Fermanagh, Northern Ireland, in the west. As 15 degrees is equivalent to 1 hour, this is a time difference of just over 38 minutes.
- Type
- Chapter
- Information
- A Journey through the UniverseGresham Lectures on Astronomy, pp. 255 - 266Publisher: Cambridge University PressPrint publication year: 2014