THE SUN.

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almost as great, as that between any two such observatories aswe have previously mentioned.

The stars, as we have stated, are so distant that no appre-ciable effect can be produced upon their apparent positions byany such change in the observer’s place. But it is found thatvery appreciable differences are seen in the apparent distancesbetween Mars and neighbouring stars, when they are measuredsoon after simset and shortly before sunrise. From thesedifferences the known amount of the observer’s displacementmay, by suitable calculations, be compared with his distancefrom Mars . That distance is consequently deduced with muchaccuracy, and, as explained in page 16, the distance of theEarth from the Sun is then also known; but with this verygreat advantage, that all the calculations made are foundedon observations taken in the same place, by the same observer,and with the same instrument.

A form of telescope, originally invented to measure the Sun’sapparent diameter, and called a Heliometer, is found to be thebest with which to take the interval between the planet and astar. It. was with a valuable instrument of this description verykindly lent by Lord Lindsay (now the Earl of Crawford andBalcarres) that Mr. Grill (since appointed Her Majesty’s Astro-nomer at the Cape of Good Hope ) made a special journey tothe Isle of Ascension, where he resided for six months in theyear 1877. In spite of many difficulties he obtained anexcellent series of observations, involving about 350 setsof measures. The locality was chosen because it fulfilled theconditions which we have described as most important. Theyear, because Mars was nearer to the Earth than it will beagain at any time during the present century. There is nodoubt that very great weight is due to Mr. Gill’s observations,and to his calculations founded upon them. He has employedall possible care in their computation, and has been ablyassisted in the accurate determination of the places of the starsused, by the principal observatories of this and of othercountries. He concludes that the distance of the Sun is verynearly 93,080,000 miles ; the probable error in excess or defectnot being more than about 127,000 miles.

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