51
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
M. Spagnulo, The Geopolitics of Space Exploration, Springer Praxis Books,
https://doi.org/10.1007/978-3-030-69125-7_6
6
GPS: The Stellar Clock
“Every clock that enables us to follow the sidereal motion of the stars reveals
itself to be a magnificient tool created by God”.
From Dava Sobel’s book Longitude, published 1995
The Earth rotates around an imaginary axis that passes through the poles, completing a full turn in 24 hours. At the equator, the circumference of the planet is
about 24,854 miles. Someone walking at that latitude moves – totally unaware – at
the speed of 994 miles per hour due to the Earth’s rotation.
In the brief time needed to walk a few steps on the ground, the Earth is rotating
at 15 degrees per hour, that is, 18 linear miles per second, while traveling around
the Sun at the speed of 62,137 miles per hour, completing a full revolution in 365
Earth days.
In a few seconds, the planet travels over 600 miles in deep space, taking along
billions of people who feel secure and stable with their feet on the ground. We are
largely aware of time passing due to the succession of days and seasons. The
seasons follow a repetitive ritual whose periodicity is due to the fact that the
Earth, while rotating on itself, tilts along its trajectory around the Sun. This
causes the angle of incidence of the Sun’s rays on the Earth’s surface to change,
moving the North Pole away from the Sun for six months and bringing it a bit
closer in the following six. The tilt also changes by two degrees over the span of
41,000 years. This how the perennial and repetitive passing of the seasons is
generated.
Since time immemorial, humans have tried to grasp the essence of time and
space and understand how these concepts influenced their lives. Sailors routinely
observed the stars and the Sun to chart their course. By measuring with a sextant
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
M. Spagnulo, The Geopolitics of Space Exploration, Springer Praxis Books,
https://doi.org/10.1007/978-3-030-69125-7_6
6
GPS: The Stellar Clock
“Every clock that enables us to follow the sidereal motion of the stars reveals
itself to be a magnificient tool created by God”.
From Dava Sobel’s book Longitude, published 1995
The Earth rotates around an imaginary axis that passes through the poles, completing a full turn in 24 hours. At the equator, the circumference of the planet is
about 24,854 miles. Someone walking at that latitude moves – totally unaware – at
the speed of 994 miles per hour due to the Earth’s rotation.
In the brief time needed to walk a few steps on the ground, the Earth is rotating
at 15 degrees per hour, that is, 18 linear miles per second, while traveling around
the Sun at the speed of 62,137 miles per hour, completing a full revolution in 365
Earth days.
In a few seconds, the planet travels over 600 miles in deep space, taking along
billions of people who feel secure and stable with their feet on the ground. We are
largely aware of time passing due to the succession of days and seasons. The
seasons follow a repetitive ritual whose periodicity is due to the fact that the
Earth, while rotating on itself, tilts along its trajectory around the Sun. This
causes the angle of incidence of the Sun’s rays on the Earth’s surface to change,
moving the North Pole away from the Sun for six months and bringing it a bit
closer in the following six. The tilt also changes by two degrees over the span of
41,000 years. This how the perennial and repetitive passing of the seasons is
generated.
Since time immemorial, humans have tried to grasp the essence of time and
space and understand how these concepts influenced their lives. Sailors routinely
observed the stars and the Sun to chart their course. By measuring with a sextant