InboxAstronomy:Hubble Stretches Stellar Tape Measure 10 Times Farther into Space Astronomers continue refining the precision of distance measurement techniques to better understand the dimensions of the universe. Calculating the age of the universe, its expansion rate, and the nature of a mysterious repulsive force called dark energy all depend on the precise distance measurements to stars and galaxies. If the astronomical yardsticks are off, the astronomical interpretation may be flawed. The most reliable method for making astronomical distance measurements is to use straightforward geometry where the 186-million-mile diameter of Earth's orbit is used to construct a baseline of a triangle, much as a land surveyor would use. If a target star is close enough, it will appear to zigzag on the sky during the year as a reflection of Earth's orbit about the Sun. This technique is called parallax. The stars are so far away that the angle of this parallax shift is incredibly tiny. An innovative new observing technique has extended Hubble's yardstick 10 times farther into our galaxy, out to a distance of 7,500 light-years from Earth.

APOD:Earth-size Kepler-186f
Illustration Credit: NASA Ames / SETI Institute / JPL-Caltech, Discovery: Elisa V. Quintana, et al.

Explanation: Planet Kepler-186f is the first known Earth-size planet to lie within the habitable zone of a star beyond the Sun. Discovered using data from the prolific planet-hunting Kepler spacecraft, the distant world orbits its parent star, a cool, dim, M dwarf star about half the size and mass of the Sun, some 500 light-years away in the constellation Cygnus. M dwarfs are common, making up about 70 percent of the stars in our Milky Way galaxy. To be within the habitable zone, where surface temperatures allowing liquid water are possible, Kepler-186f orbits close, within 53 million kilometers (about the Mercury-Sun distance) of the M dwarf star, once every 130 days. Four other planets are known in the distant system. All four are only a little larger than Earth and in much closer orbits, also illustrated in the tantalizing artist's vision. While the size and orbit of Kepler-186f are known, its mass and composition are not, and can't be determined by Kepler's transit technique. Still, models suggest that it could be rocky and have an atmosphere, making it potentially the most Earth-like exoplanet discovered so far ...

APOD:Red Moon, Green Beam
Image Credit & Copyright: Dan Long (Apache Point Observatory) - Courtesy: Tom Murphy (UC San Diego)

Explanation: This is not a scene from a sci-fi special effects movie. The green beam of light and red lunar disk are real enough, captured in the early morning hours of April 15. Of course, the reddened lunar disk is easy to explain as the image was taken during this week's total lunar eclipse. Immersed in shadow, the eclipsed Moon reflects the dimmed reddened light of all the sunsets and sunrises filtering around the edges of planet Earth, seen in silhouette from a lunar perspective. But the green beam of light really is a laser. Shot from the 3.5-meter telescope at Apache Point Observatory in southern New Mexico, the beam's path is revealed as Earth's atmosphere scatters some of the intense laser light. The laser's target is the Apollo 15 retroreflector, left on the Moon by the astronauts in 1971. By determining the light travel time delay of the returning laser pulse, the experimental team from UC San Diego is able to measure the Earth-Moon distance to millimeter precision and provide a test of General Relativity, Einstein's theory of gravity. Conducting the lunar laser ranging experiment during a total eclipse uses the Earth like a cosmic light switch. With direct sunlight blocked, the reflector's performance is improved over performance when illuminated by sunlight during a normal Full Moon, an effect known as the real Full Moon Curse.