It will be a challenge to catch the slim waning crescent moon near the planet Mercury in the glow of dawn on Friday morning, July 25, 2014. Look east. You’ll see the dazzling planet Venus, with the moon below. Quite close to the sunrise point the horizon, about an hour or so before sunrise, you’ll find Mercury – surprisingly bright for being so low in the sky.
An unobstructed horizon and binoculars will come in handy for seeking out the old waning crescent moon and Mercury before sunrise on July 25. The exact rising times of the moon and Mercury vary worldwide, so we recommend using a sky almanac to find out when they’ll rise into your sky. Keep in mind that the rising times presume a level horizon.
Mercury, the closest planet to the sun, and Venus, the second-closest planet, are called inferior planets because their orbits are inside that of Earth. The planets that orbit the sun outside Earth’s orbit (Mars, Jupiter, Saturn, Uranus and Neptune) are called superior planets.
The closer that a planet resides to the sun, the faster that it travels in its orbit. Mercury goes around the sun at mean velocity of 47.9 kilometers (29.7 miles) per second. Venus’ mean velocity is 35 km (21.8 miles) per second while that of Earth is 29.8 km (18.5 miles) per second.
However, Mercury has the most eccentric – oblong – orbit of any solar system planet, so its orbital velocity varies from 58.98 km/sec, when it comes closest to the sun in its orbit, to 38.86 km/sec, when it’s farthest from the sun in its orbit.
Venus, on the other hand, has the least eccentric orbit of any solar system planet – meaning Venus’ orbit is close to circular. Thereby, Venus’s orbital velocity only varies from 34.79 to 35.26 km/sec.
As far our planet Earth is concerned, it has the third-smallest orbital eccentricity, after the planets Venus and Neptune. Our orbital velocity only varies by 1 km/sec throughout the year, from 29.29 to 30.29 km/sec.
Bottom line: On Friday – July 25, 2014 – you’ll need a clear sky and unobstructed horizon to catch a sliver of the old moon plus Venus and Mercury. If you spot ‘em, congrats!