The Increasing Distance To The Moon

When, back in Chapter 2, we looked at the fundamental processes by which eclipses eventuate, we noted that the angular diameter of the Moon as seen from the Earth is almost precisely the same as that of the Sun. This is a quite remarkable coincidence. There are small cyclic variations in those apparent sizes because the Earth— Moon distance changes as the latter moves between perigee and apogee, and the Earth—Sun separation alters as the former moves between perihelion and aphelion. Nevertheless it seems a staggering coincidence that the angular diameters of the Sun and the Moon are so similar.

If the dimensions of either Moon or Sun were a little bit different, then the stringent eclipse conditions would collapse. If the Moon were slightly further away then no total solar eclipse could ever occur. Conversely, if it were slightly closer then eclipses would occur more frequently, and we would have added opportunities to wonder at them.

In fact the Moon was closer to us in the past. And if you happen to read these words precisely one year after I typed them, then, the Moon will have receded from the Earth by about an inch and a half.

The value I have just given for the increasing separation is derived directly from lunar laser ranging experiments. Between 1969 and 1972 the Apollo astronauts left several retro-reflectors on the lunar surface, these acting similarly to the glass "cat's eyes" inserted along the central line of a road, reflecting back the light from an advancing car's headlamps. The retro-reflectors installed on the lunar surface are similar devices, although rather more sophisticated, shaped like the corner of a cubic prism (see Figure 6-1).

FIGURE 6-1. The laser retro-reflector array left on the surface of the Moon in the Apollo 14 mission in 1971. A hundred separate corner-cube prisms are used in this device. Over the past 30 years it has been used to reflect laser pulses back to observatories on Earth, making it possible to monitor the slow drift of the Moon away from our planet.

FIGURE 6-1. The laser retro-reflector array left on the surface of the Moon in the Apollo 14 mission in 1971. A hundred separate corner-cube prisms are used in this device. Over the past 30 years it has been used to reflect laser pulses back to observatories on Earth, making it possible to monitor the slow drift of the Moon away from our planet.

By firing short laser pulses towards these through a large telescope and measuring the time it takes for a few of the transmitted photons to be reflected back to the Earth, physicists can measure the round-trip time: slightly over two and a half seconds. Knowing the speed of light, the experimenters can determine rather accurately the current distance to the Moon. Over three decades of such trials they have shown that the recessional speed of the Moon is about an inch and a half per year.

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