Galaxies And Jack Havig

In 1951, Jack Havig supposes that, while he time travels, it is gravity that keeps him in the same place on the Earth's surface although:

"'...this planet is spinning on its axis, and whirling around a sun which is rushing through a galaxy...'"
-Poul Anderson, There Will Be Time (New York, 1973), IV, p. 38.

He could also have mentioned the motion of the galaxy. Other galaxies were recognized in 1925, Poul Anderson was born in 1926, cosmic expansion was discovered in 1929 and Jack Havig was born in 1933. See Significant Dates. I mention this because, yes, Jack would have known about galaxies in 1951 but, no, we should not take such scientific knowledge for granted. Galaxies had been recognized only twenty six years earlier. The parents both of Poul Anderson and of Jack Havig had been born and grown up in a world where it was thought that the galaxy was the universe.

The motion of Earth through space can affect inter-temporal communication if not also inter-temporal travel. In Gregory Benford's Timescape, "the past is in the sky." A tachyonic beam aimed at an earlier year must be aimed at the point in space where Earth was in that year. Alternatively, James Blish's Dirac messages can be received at any point in space-time. Poul Anderson's Time Patrolmen must send written or typed messages in tiny time shuttles. Temporal communication is not time travel but is a subsidiary concept.