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The particle horizon is the boundary between two regions at a point at a given time: one region defined by events that have already been observed by an observer, and the other by events which cannot be observed at that time. It represents the furthest distance from which we can retrieve information from the past, and so defines the observable ...
For a tower with a height of h L =100 m, the horizon distance is D L =35.7 km. Thus an observer on a beach can see the top of the tower as long as it is not more than D BL =40.35 km away. Conversely, if an observer on a boat ( h B =1.7 m) can just see the tops of trees on a nearby shore ( h L =10 m), the trees are probably about D BL =16 km away.
A typical example of long-distance observation. The Tatra Mountains as seen from the Łysa Góra, in southeast Poland, at a distance of about 200 km (120 mi).. Long-distance observation is any visual observation, for sightseeing or photography, that targets all the objects, visible from the extremal distance with the possibility to see them closely.
Sighting the height of a landmark can give a measure of distance off and, held horizontally, a sextant can measure angles between objects for a position on a chart. [1] A sextant can also be used to measure the lunar distance between the moon and another celestial object (such as a star or planet) in order to determine Greenwich Mean Time and ...
Rather, the conformal time is the amount of time it would take a photon to travel from where we are located to the furthest observable distance, provided the universe ceased expanding. As such, η 0 {\displaystyle \eta _{0}} is not a physically meaningful time (this much time has not yet actually passed); though, as we will see, the particle ...
Despite being an integral over time, this expression gives the correct distance that would be measured by a set of comoving local rulers at fixed time t, i.e. the "proper distance" (as defined below) after accounting for the time-dependent comoving speed of light via the inverse scale factor term / (′) in the integrand.
If the height h is given in feet, and the distance d in statute miles, d ≈ 1.23 ⋅ h {\displaystyle d\approx 1.23\cdot {\sqrt {h}}} R is the radius of the Earth, h is the height of the ground station, H is the height of the air station d is the line of sight distance
(The numerical value of the Hubble length in light years is, by definition, equal to that of the Hubble time in years.) The Hubble distance would be the distance between the Earth and the galaxies which are currently receding from us at the speed of light, as can be seen by substituting D = cH −1 into the equation for Hubble's law, v = H 0 D.