Earth’s orbit is carrying it to a rendezvous with a planetary neighbor in August that is virtually unprecedented in all of human history. On Aug. 27, Earth and Mars will share what in celestial terms could be considered a face-off when the planets pass within 34.7 million miles of each other in a phenomenon astronomers call opposition — when Earth reaches a point on a direct line between the sun and one of the other planets. Although opposition is a routine event — it occurs between Mars and Earth about every two years and 50 days — this particular one carries a special distinction. Astronomers have calculated it will be the closest proximity of Earth and the red planet in 73,000 years.