Five Planets Visible

For the first time in a decade, if you are an early riser, you will be able to see all five of the naked eye planets in the sky. For the next month, just before sunrise, Mercury, Venus, Mars, Jupiter, and Saturn will be visible in the sky to the east. This sort of alignment can only be seen just before sunrise or just after sunset because Venus and Mercury can never be far from the Sun in the sky. Check out this post from July 16, 2014.

Remember, planets do not twinkle as compared to stars. So you should be able to easily spot the planets in the sky compared to the stars. Just for note, Mercury will be the planet closest to the horizon and Jupiter will be the farthest. The order of the planets in the sky will be Mercury, Venus,  Saturn, Mars, and Jupiter. If you have a powerful enough telescope, you also might be able to see Pluto near Mercury.

One other cool observation is beginning around the third quarter phase, the Moon will also be visible along with the five planets. As the Moon ages towards the New Moon, it will be among the planets in the sky.

Note: if you are really observant, you will be able to see a sixth planet. Comment below if you know which planet and why you will be able to see it.

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.

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.

Conjunction and Opposition

Sometimes, there are unique occurrences when planets align themselves with the Earth and the Sun. We actually have specific names for these occurrences: opposition and conjunction.

 

Opposition occurs when the Sun and the planet are in opposite directions in the sky, i.e. 180° apart in the sky. The Full Moon is also an example of opposition. Venus and Mercury, however, can never be in opposition. In my post about the morning and evening star, we know that Venus is never farther than 47.8° and Mercury is never more than 27.8° from the Sun.

 

Conjunctions happen to all the planets. The best way to think of it is when a planet and the Sun are in relatively the same direction in the sky. When the outer planets (Mars, Jupiter, Saturn, Uranus, and Neptune) are in the same direction as the Sun, we just refer to it as a conjunction. However, since Venus and Mercury can actually be at conjunction at two different points in their orbits, we need to specify their two conjunctions. When the inner planet is closest to the Earth, we call that its inferior conjunction because it is closer to the Earth than the Sun and when its at its furthest point, we call that the superior conjunction.

Planets can also be at conjunction with other planets. A good example is now visible in the night sky. Right now for a few days, Jupiter and Venus are near each other in the sky, and depending on your telescope or binoculars, are in the same field of view.

Venus and Jupiter in conjunction, with the full moon to give scale.

Via APOD

Composite Image Credit & Copyright: Wang, Letian

 

 

Planetary Alignment

Every once in a while, a story comes out that a planetary alignment will wreak havoc on Earth and the end of the world as we know it will occur. The most recent example was posted by a friend of mine on Facebook. It basically tells us that a planetary alignment will cause a massive 9.8 earthquake in California on May the 28th. Here is a link for the story if you wish to read it. I admit that I didn't read it because I have one thought on stories like this: they basically come from the back end of a male cow.


These stories are bogus. They are a waste of electrons on the internet. People that don't know science are taken in by these articles because of the gloom and doom that they predict. I'm going to explain why these stories are false.


1. The Sun is the overwhelming largest mass in the Solar System, and by a large margin, contributes the largest component of the force of gravity on the Earth. All the other planets, moons, asteroids, comets, minor bodies, dust particles, alien lifeforms, nanobots, whatever, in our Solar System combined do not contribute much to the force of gravity felt on Earth. Remember, the force of gravity is related to the mass divided by the distance squared (higher the mass, higher the gravity; closer the mass, the higher the gravity as well). Remember, that the Sun contains over 99% of all the mass in the Solar System, and that there are only six objects in Solar System closer to Earth than the Sun (Mercury, Venus, the Moon, Mars, Phobos, and Deimos). They do not have enough mass combined to do anything to our planet. In fact, the only other mass that has an significant affect on Earth is the Moon, and all it does is make water slosh around (see the post on tides).


2. The planets themselves cannot align themselves in such a way that they all are pulling on Earth in the same direction, even if the gravity was strong enough. All the planets are inclined differently to the Sun's equator, and in turn, to the Earth's orbital plane. One planet might be above the Earth's orbital plane, while another below it. Note that it is possible that the planets might all be in the same orbital plane as the Earth as they orbit the Sun, but this is highly unlikely due to all the motions of the planets. But again, it is extremely unlikely that they all would happen to be in the same orbital plane as the Earth, and even then, look at the first point.


I am betting that if you read my blog, you know that these stories are blatantly wrong. However, if by some chance you believe them, just understand, planetary alignments will not hurt the Earth in anyway. I'd be more worried about the people living on the planet.

General Relativity and Astronomy

Previously, we discussed how mass can curve space(time) due to general relativity. Why is this important?

The curvature of mass leads to interesting phenomena. The first is it causes the perihelion of a planet orbiting the Sun to precess. Secondly, it causes light actually to bend - yes, gravity affects light - and this leads to really weird stuff.

Let's look at the first one. The best example of the precession of a planet at it orbits the Sun is the path of Mercury. This was discussed back in the post about Mercury and General Relativity. The highlight of the discussion was that as Mercury orbits around the Sun, at perihelion, Mercury is in the deepest part of the gravity well created by the Sun. As it continues to orbit, each successive perihelion moves farther ahead in its orbit. The perihelion of Mercury was noticed in the mid 1800s, but was thought to be caused by an inner planet. But after Einstein's Theory of General Relativity was developed, the equations were able to show why Mercury's orbit precessed around the Sun. Everything that orbits around another body shows this precession, with the amount of precession dependent on the mass of the central body.

The second one seems a little weirder. From Newton's equation for universal gravitation, we see that the force of gravity is dependent on mass. However, light and all electromagnetic radiation are massless. So how does gravity bend light?

In a nutshell - gravity wells.

If a star were behind the Sun, in Newtonian gravity, we would not be able to see it, because gravity only affects objects with mass. We would see something like this.

However, because of General Relativity, light will bend in the presence of a gravitational field. The light from the star will curve around the Sun and we can see it from Earth.

This was actually proven by Sir Arthur Eddington. In 1919, there was a solar eclipse that he observed and photographed. When the pictures were analyzed, they could see the effect of gravity on stars. This analysis only worked during an eclipse because otherwise, the Sun would be too bright and wash out the background stars. Eddington gave physical proof that General Relativity was correct!

Positive and Negative Image of Solar Eclipse of May 1919

Image Credit:



F. W. Dyson, A. S. Eddington, and C. Davidson

This phenomena of light bending around masses also is used to search for exoplanets. As a planet passes in front of star, that planet can bend the light towards us. This process is called lensing. Not only does it allow the light to reach us even if the source is behind the mass, it can also magnify the light, making it brighter. Most of the gravitational lensing seen are galactic in nature.

The curved arcs are the lensed (background) object. The centers are the lenses bending the light.

Image Credit:

NASA/Hubble Space Telescope

Einstein's Cross (Gravitation Lensing) - Quasar being lensed by a central dim galaxy

Image Credit:

ESA/Hubble/NASA

Einstein Ring - When the background object is perfectly aligned with lensing object and the Earth, a complete ring can be created

Image Credit:

NASA, ESA, A. Bolton (Harvard-Smithsonian CfA) and the SLACS Team

Hubble Site