Twinkling Stars and Static Planets

Twinkle, Twinkle Little Star
How I wonder what you are.




Everyone remembers this nursery rhyme. The question is why do stars twinkle? And how come planets do not?




The short answer is that it has to do with the apparent diameter of the star and the planet. Stars are generally so far away that they have no apparent diameter, so that when light from the star enters our atmosphere, the light is easily diffracted due to different pockets of air. Since the width of the light is so small, these diffractions make it seem as if the star's light is constantly blinking in and out as the light is diffracted away from our eye as the light travels through the atmosphere.




For planets, however, they do have an apparent diameter, albeit much tinier than that of the Sun or the Moon. But this size is enough that when light from the planet travels through different pockets of air in the atmosphere, only a small portion of the light is diffracted away from our light of sight, so the brightness of the planet does not waver, and therefore does not twinkle.

Supermoon Lunar Eclipse

On September 27th, we experienced an event that does not happen very often, a supermoon lunar eclipse. We have had a supermoon before, and I've talked about it (see Supermoon). A supermoon is just the moon at perigee. However, this time, we had a supermoon occur when we had a lunar eclipse.

Lunar eclipses aren't all that rare (they happen once every six months), but to have one occur when the moon is at perigee is a sight to behold. The moon appears the largest and the lunar eclipse at this time is actually the shortest total lunar eclipse to occur. It is the shortest because the moon is at its closest, so it is actually moving at its fastest. Many lunar eclipses can last a few hours, but totality for this eclipse was only an hour and a half.

If you want to know more about lunar eclipses, see here.

Autumnal Equinox

This past September 23rd, the northern hemisphere experienced the beginning of our fall season.

In terms of astronomy, what does this mean?

In the sky, if we could see the ecliptic, the path the Sun travels on as it appears to cross the sky, the point of the autumnal equinox occurs when the Sun crosses the celestial equator as it travels south on the ecliptic. This marks the beginning of autumn for all of us in the northern hemisphere, and the beginning of the spring for those in the southern hemisphere.

Another way to look at it, is that the Sun is directly overhead at the equator, which means that the Earth itself is tilted in the plane perpendicular to the Sun. Neither the northern hemisphere or the southern hemisphere is tilted towards the Sun. But at this point, the southern hemisphere will begin to receive more sunlight as the southern hemisphere is beginning to be tilted more towards the Sun.

Ions

An ion is a particle that is similar to an atom but has more or less electrons than protons in the nucleus. In most cases, ions are present in compounds that have a metal and a non-metal (like NaCl, common table salt), but can also be present in hot gases or plasmas, like the Sun.


If an ion has more electrons than protons, i.e. it is negatively charged, we call that an anion. In the above example (NaCl), the chlorine ion has one more electron than proton (18 electrons vs. 17 protons) and therefore, has a charge of -1 e (-1.602e-19 coulombs). If the ion has less electrons than protons, it is a cation. The sodium ion in NaCl has one less electron than proton, so has a charge of +1 e.


Why are ions are important? They are found in the spectra of stars, in nebulae, and as stated above many compounds that we use everyday. In the Sun or any star, the gas is so heated, that it can strip electrons from the atoms, ionizing the gas, converting it into a plasma.


There are some elements that do not ionize easily. We call these elements inert and they are found on the far right of a periodic table. These are the noble gases: Helium, Neon, Argon, Krypton, Xenon, and Radon. This does not mean that they cannot be ionized. Under normal temperatures and pressures, the outer electrons are not readily removed from their orbits. However, under extreme temperatures and pressures, electrons can be stripped from the outer shells. In fact, the alpha particle is the bare nucleus of a helium atom with a charge of +2 e. We find helium ions in the core of the Sun, as it is the final product of the proton-proton chain. Electrons do not easily combine with the bare nucleus to form a stable helium atom.


Next time, we will learn about ion engines and how they may be a future propulsion system for solar system travel.

Bright Spots on Ceres

I have previously posted about asteroid/dwarf planet Ceres before. The Dawn Spacecraft recently arrived at Ceres and began a comprehensive study of the dwarf planet that has never been done before. One of the strange things that Dawn found on Ceres were bright spots that confounded scientists at first. What were they?

Via NASA/Dawn

 

The bright spots were discovered in a crater now known as Occator, an 80-km diameter crater at 19.5° latitude on Ceres. They are called faculae which means “bright spots”. Faculae are more commonly known as the bright regions on the Sun surrounding a sunspot. Where do the faculae on Ceres come from?





One theory is that they are ice spots in the crater that are reflecting sunlight or salt deposits left over after salty water on the surface evaporated away. These spots might have come from Ceres actually having a dusty surface, and minor impacts on the surface exposed the underlying ice or salt.



Another theory, which is more widely accepted, is that the spots are actually ice geysers or cryovolcanoes (volcanoes that spew ice rather than lava). This is believed to be the true cause as Dawn has seen haze above the spots.





We will probably learn more as Dawn continues its reconnaissance around Ceres. And if these are ice features, Ceres could be a great place to build a future way station in the Solar System.

 

Blue Moon

Tonight, July 31st, a rare occurrence will happen. You can say that it is something that happens once in a blue moon, because it is a blue moon.


What do we mean by a blue moon? There are actually two definitions of what a blue moon is.

1. A blue moon is the third full moon in a season where four full moons occur. Most times, there are only three full moons in one season, but on the rare occasion when a full moon is at the beginning of the season, the season may have four.
2. A blue moon may also mean the second full moon in a calendar month. The full moon of tonight is this type of blue moon.

Why do blue moons occur and why are they rare? If you go back to when we discussed the lunar cycle and the synodic period of the moon, we know that the time from one full moon to the successive full moon is approximately 29.5 days. Calendar months last anywhere from 28 days to 31 days. Therefore, if a full moon is on the first of a month, that month will have a blue moon (second definition). The seasons are, on average, a little over 90 days. As stated above, if the full moon occurs on the solstice or equinox (or a day after), it is possible to have four full moons in the season.


If you miss the blue moon tonight, don't worry. There will be another blue moon on May 21, 2016. This will be the third blue moon in the northern spring (southern fall) of 2016.

Travelling to the Planets

Recently, after over 9 years of travel through the Solar System, New Horizons arrived at Pluto and flew through the system. I've talked about the probe before and you can click the link above to see my previous post. New Horizons has already taken some amazing images of the system, and we will be getting more images as time goes on.





However, the amazing feat is not that we are taking images of Pluto and its moons, but that New Horizons actually made it to Pluto. Many things could have gone wrong: it could have been hit by an unknown asteroid or comet, it could have inexplicitly lost power, or a biggie: the trajectory could have been miscalculated.





When we go to any body in the solar system, we cannot aim directly for it. Much like a quarterback leads his receiver in American football, the probe must be aimed ahead of the planet or body and must arrive at the same place and time as the planet or body.





In the image down below, if we were to aim a spacecraft at Mars where it is now located, by the time we reached it, Mars would be farther ahead in its orbit. To reach Mars, we must aim to where Mars will be in about six months (the minimum time it takes to reach Mars from Earth). For longer travel times, we must aim farther ahead in the Martian orbit.

To reach Pluto took a little more doing. We had to know how long it would take to reach Pluto, and aim New Horizons to where it will be then. Knowing that New Horizons will reach the Pluto system in nine years and how fast Pluto is moving on its orbit, engineers and scientists were able to determine where New Horizons should be aimed towards in nine years. They also had to worry about avoiding the other planets and moons, and had to hope that New Horizons wouldn't encounter any stray asteroids or comets that we did not know about.

The Earth is in relatively the same location on its orbit as it was nine years ago.