The European Space Agency (ESA) is Europe’s gateway to space. Its mission is to shape the development of Europe’s space capability and ensure that investment in space continues to deliver benefits to the citizens of Europe and the world.
Find out more about space activities in our 22 Member States, and understand how ESA works together with their national agencies, institutions and organisations.
Exploring our Solar System and unlocking the secrets of the Universe
Go to topicProtecting life and infrastructure on Earth and in orbit
Go to topicUsing space to benefit citizens and meet future challenges on Earth
Go to topicMaking space accessible and developing the technologies for the future
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As Earth orbits the Sun, we see an apparent shift in the positions of stars. Known as parallax, this movement is larger for nearby stars and smaller for more distant stars.
Measurements of these stellar movements can be used to determine the distances to the stars. The baseline used for the triangulation is fixed by the size of Earth's orbit around the Sun: the average Earth–Sun distance, or Astronomical Unit (AU), is 149 597 870 700 m.
This illustration shows the shift in a star's position with respect to the distant stellar background between two observations that are separated by six months – for example, the first one in January and the second one in July. The extent of the parallax has been exaggerated for illustration purposes.
Even for the stars that are closest to Earth, the annual shift due to parallax is extremely small, requiring high-precision instruments. The first stellar parallaxes, for a handful of stars, were measured by Friedrich Bessel, Wilhelm Struve and Thomas Henderson in the late 1830s. In more recent times, parallaxes for over 100 000 stars have been measured, and ESA's Gaia mission will measure them for more than a billion stars.