StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

JUNE 2009

JUNE STAR MAP | STARWATCH INDEX | MOON PHASE CALENDAR

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668 JUNE 7, 2009: Ringing in the Summer: We have now entered the time of the long days and short shadows, and I am very happy about that. Not only will school be over in just a few weeks, but because Labor Day falls on September 7, teachers in my school district won’t be reporting back to work until the first. It will seem more like an old fashioned summer recess, similar to when I first started teaching some 37 years ago. Although summer solstice will not occur until June 21 at 1:45 a.m. EDT, the sun and Earth will be engaged in some interesting annual milestones prior to and after the solstice date. That is because of the Earth’s elliptical or oval-shaped orbital path, and its axial tilt of 23-1/2 degrees from the vertical to its orbital plane. The former causes Earth’s orbital speed through space to speed up and slow down as we approach and recede from the sun. The latter, Earth’s axial tilt, causes the sun’s daily change in altitude to be greatest around the equinoxes of March and September. This slows the sun’s eastward progression among the stars. These factors put the longest day of the year, June 21, out of synch with the earliest sunrise and latest sunset dates by nearly two weeks. On June 14 we will experience the earliest sunrise (4:31 a.m.), followed by the high sun of the summer solstice on June 21. The latest sunset will happen almost a week after the solstice on the 27th (8:33 p.m.). Our greatest distance from the sun, aphelion, doesn’t take place until July 4, voiding the misconception held by most people that distance plays the critical role in why we experience seasonal changes. One nice consequence of the latest sunsets occurring after the June solstice date is that it seems to prolong the extended summer days for just a little while longer. That’s because we are usually awake for summer sunsets, but still happily in dreamland for summer sunrises. Happy solstice, happy summer!

669 JUNE 14 2009: Aristarchus and the Earth-Sun Distance: The Greek scholar Aristarchus of Samos (310-230 BC), a proponent of the sun-centered theory of the universe, devised an ingenious experiment to find the relative distance of the Earth from the sun by using the phases of the moon. Believing that the orbital path of the moon was a circle, and that it traveled at a constant speed with the sun’s rays diverging outward from its source, Aristarchus conceived his basic experiment. He knew the moon was a sphere from watching its progression of phases and noting that the changing sunrise/sunset locations (terminator) on the moon were curves. Aristarchus also realized that the terminator could only appear straight when the moon formed the vertex of a right angle with the sun and the Earth. Aristarchus reasoned that because the sun’s rays were diverging, the time intervals between last and first quarter moons should be shorter than the time periods between first and last quarter phases. These time differences between the quarter moons could be converted into angles since the phase period of the moon, 29.5 days, was understood. Knowing the angles generated by the quarter moons allowed for a geometric construction of the problem and the relative distance of the Earth from the sun to be calculated. Even though today we know that most of Aristarchus’ assumptions were wrong, and that the moment of the quarter moon cannot be observed with any accuracy, the experiment was performed and Aristarchus obtained an Earth-sun distance of 20 times the Earth-moon distance. Although this figure was still some 20 times too small, it proved that the sun was a lot farther away than the moon. You can observe the last quarter moon on the morning of Tuesday, June 16 (about nine hours too late) and the first quarter moon on the evening of Sunday, June 28 (about eight hours too early).

670 JUNE 21, 2009: Ready Your Telescope for Apollo 11: With the warmer summer weather upon us, you might have the urge to dust off that telescope stashed in the closet for the past 10 years. July 20 marks the 40 anniversary of the Eagle landing and the first moonwalk by Neil Armstrong and Buzz Aldrin. On Sunday, June 28, the southwestern region of Mare Tranquillitatis, where the landing and walk actually took place, will be in a prime position for viewing. See the online map associated with this week’s StarWatch at www.astronomy.org. When you dust off your telescope, remember that it is important not to dust off the optics. They usually contain special coatings that can be scratched if wiped too vigorously. For closed telescopes with a lens upfront, a can of “air” used to clean computer components can blow away most unwanted particles. Just remember to hold the can upright to prevent any propellant from escaping onto the lens. Also make sure that your wide field finder scope, the smaller telescope attached to your tube, is properly aligned with the main telescope. You should perform this alignment procedure in the daytime by viewing a distant object on the horizon through your main scope. Adjust the screws of the finder scope mounting brackets so that the crosshairs of the finder are centered on the same feature as the main telescope. If your telescope has an equatorial mounting system (check your instructional manual), the polar axis should be pointed towards Polaris, the North Star. Polaris can be found by following the two (lowest) Pointer Stars of the Big Dipper, now in the 11 o’clock position, just after nightfall. Aiming the polar axis towards the North Star will allow you to track the moon by only moving one axis, making for a more enjoyable “ride” across the lunar surface.

671 JUNE 28, 2009: Apollo 11: To the Moon: Nine seconds before liftoff, thousands of gallons of highly refined kerosene began to mix with liquid oxygen as the center engine of the Saturn V rocket was lit. The outer four engines ignited in pairs within 300 milliseconds of each other so that the 36.3-story, 6.6 million pound rocket would not topple over as it rapidly built to its 7.61 million pounds of thrust. Forty years later, it still remains the largest rocket to be used successfully in spaceflight, and it was the loudest machine ever constructed. At T=0, locking bolts released the Saturn V, and it ponderously lifted off its launch pad at Florida’s Cape Canaveral. Twelve minutes later, after exhausting the fuel of its first two stages, the third stage, command, service, and spacecraft lunar module adapter (SLA-where the lunar module was stowed) reached Earth orbit. After one and a half revolutions, the hydrogen-oxygen burning third stage was fired for translunar injection, placing the astronauts on a course headed towards the moon. One half hour later, the command and service module package (CSM) separated from the third stage and SLA, and thrust forward, turned around, and came back to dock with the lunar module (LM). The LM was pressurized and then extracted from the SLA. The initial push forward of the LM-CSM was provided by four springs inside the SLA. Additional help came from the service module thrusters maneuvering the CSM-LM far enough away from the third stage so that it could perform a dump of its hydrogen and oxygen fuel. This would propel it past the trailing side of the moon and into an orbit around the sun. Except for course corrections during the coasting time to the moon, the five-story CSM-LM package was in the “barbecue” mode, rotating slowly to maintain thermal equilibrium. These events got the Apollo 11 astronauts headed towards the moon.

[June Star Map]

[June Moon Phase Calendar]