How do we know that Sun is a star?

today I will shortly explain how is it that we know that Sun is a star and why it is not so trivial.

When you look up on the clear sky in night you see dots and they are said to be basically the same thing as our close Sun. They definitely shine, but not so much, they are not very warm, light up in different time than Sun, for some reason twinkle and are way smaller, even with binoculars they are still dots.

Hell yeah, they are quite small dots! Even if you take our best telescopes you will always have them so tiny! They are “point source of light”, which means that they are simply so small that from our view and practical purposes they are dimensionless.

They are huge though, most a bit smaller then Sun but still big, but so far away that they seem only as points. If even now we can not see the star’s surface as anything else than a dot, how do we know that it is the same thing as Sun?

The thought of Sun being star was there for some time. I found that already guys in ancient Greece thought that, the same idea came to the famous row of astronomers like Copernicus, Galileo, Kepler, Newton and others but they still did not have an access knowledge that would prove it.

In 1838 Friedrich Bessel measured the distance to star for a first time without considering anything about it (I guess he might have used stellar paralax but I am not sure). He found that the distance is huge, as we know today and soon we calculated that these dots are actually about as bright as Sun, also with astronomical spectroscopy scientists found out that what is happening there is also happening here. To make it clear, we do not have a “close up” image of any star, nor do we have image of any exoplanet.. for now that is simply impossible, still we can admire the cool Jupiter that Juno is taking pics of! (it is literally porn for eyes ­čśë )


Polaris won’t be North Star forever

today I will write about precession and what effect it has on our sky.

Polaris is the North Star or Pole Star. This means that it is close to the celestial pole. Celestial pole is a point created by expanding the Earth’s axis of rotation and piercing through the celestial sphere which is imaginary “area” where stars sit. (For Earth based observations you do not really need to take into account that the stars are in completely different distances from us)

Precession is shown by the circle on the top.

But Polaris was not always the North star. For example when the Great Pyramid of Giza was build aobut 4600 years ago, there were two shafts from the tomb added. One points towards some random stop in the sky and the other one to Polaris.. oh wait but 2600 BC it pointed to Orion’s Belt and the star Thuban in the constallation of Draco. Orion was in Egyptian mythology connected to the god of dead Osiris and Thuban used to be the star closest to celestial pole. WHY?

Orange circle shows how the celestial pole’s position will change during the next years


Because of precession. That is an effect on Earth by Moon and Sun. The same way as gyroscope creates a kind of cone shape with it top, Earth also rotates like this but very slowly, it takes about 26000 years to rotate once. This type of precession is also called axial precession.

On the southern hemisphere the South Star is Sigma Octantis. It has very high magnitude so it is barely visible and not very good for naked eye observation. This will of course also change in the next hundreds of years.

Because of precession astronomers have to update every 50 years the positions of stars and objects, right now we are in what is called J2000 epoch and the next one will be J2050.


1st picture: By NASA, Mysid – Vectorized by Mysid in Inkscape after a NASA Earth Observatory image in Milutin Milankovitch Precession., Public Domain,
2nd picture: By Tau╩╗olunga – self, 4 bit GIF, CC BY-SA 2.5,


Orbital period


in today’s short post I will write about orbital period of planets, more accurately synodic and sidereal period.

In the post about year and how difficult it is to determine how long it is, I mentioned that there are some ways you can measure the time it takes for planet to orbit star.

Sidereal period is the time it takes for Earth or other object, orbit once with respect to distant stars.

Now distant stars are great because they tend to be on the same spot most of the time. For example on the Voyager plague there is a map to show the position of distant pulsars, why? Because such things are stable, easy to see and far away. For year we use stars in Milky Way which is still fine, most move by fractions of arcseconds every year which is something you can not notice with eye and has some effects in thousands of years.

Sidereal period of Earth around the Sun is 365.25636 days. (I wonder if you could talk about something like sidereal period of Sun around the center of Galaxy, probably yes)

Synodic period is about two bodies orbiting Sun for example. It is the time that it takes for the two objects to get to same position. So if Mars and Earth are right behind each other (which is called opposition), synodic period is the time it takes for it to happen again. Now of course both planets orbit and the faster one (the one closer to Sun) always has to make at least one revolution. When that happens it just needs to catch up with the slower planet. With this simple thought you can come up with equation that lets you calculate the synodic period:


(lower case p is the sidereal period of the object with longer period)

Thats about it for know, enjoy your winter holiday while/if you still have it!


How long is a year actually?

today I will write about a year. The thing is that as in many other subjects when you look down into the simplest things you might find that they are not as simple as they seem. So how long is year? 365 days? 366 days?

You very well know that every 4 years we have 1 extra day in February. You might also know that this is because year is not 365 days long exactly but roughly 365.25 .. its important to say roughly because it is not perfectly true and it matters how you define one year.

First lets see how we define one day. One would say that it is the time that it takes for Earth to rotate once. Problem is that we need to define some object to compare it to, some ground, some reference point. It might be the Sun, but Sun is too close and since we go around it, this would change the length of the day.

Sidereal day is the day that is defined as a rotation of Earth around its axis compared to very distant stars that are relatively stable. 23.9344699 hours… that is pretty close to 24 that we use, but it is not what we use.

The thing is that we decided to use what is called solar day, which is in fact compared to Sun. As Earth rotates around its axis, it also rotates around Sun, which makes the solar day different length.

This is how the effect looks like. You need to turn Earth n.2 by little bit more since it moved around the Sun too.

Problem is that the length of solar day changes since our orbit is sligthly elliptical and when we are closer to Sun we are faster which means that the solar day is shorter and there is more time needed for the same spot to face Sun again. This effect adds up to almost 365.25 solar days in a year. If it was so simple we could just add one leap day every 4 years to make up this 0.25 difference but it is actually 0.242181 which makes difference over time.


Julian calender ran with 0.25 for a long time but after about 1500 years it was already 10 days behind of the real date and Christians wanted to predict Easter exactly so they changed on Gregorian calendar. This calendar is the same, except that if the year is divisible by 100 it wont be a leap year, though if it is also divisible by 400 it will be a leap year. This almost fixes the problem, though every 3216 years one day is still off from the real time. Yup. Check out this video to see how we can improve this slight mistake:

So thats it. But you can not really capture the length of year or day since it changes all the time (effect of other planets and what happens on Earth). Check out this video which I used mostly as a source, it has got cool animations that will help you understand it:


Rosetta and OSIRIS-REx

today, as promised I will look upon two missions that has to do a lot with small stuff flying around the Solar System.

Now I said stuff because Rosetta is a mission for comet and OSIRIS is mission for asteroid.

Rosetta is a mission that was launched back in 2004 by ESA which is European organization. It went for the comet 67P or also called Churyumov-Gerasimenko which kinda looks like duck:

Comet 67P on 19 September 2014 NavCam mosaic.jpg

Ok, fine, it does not but look here.. from this photo I would say that it is cat with huge tumor on back.

It went with Philae which is a lander module. It took 10 years to get there. It visited two other asteroids and went around Mars.

After some small changes it went to orbit around the comet even though it has escape velocity of 1 m/s.

Then it deployed Philae in 2014 but harpoons that should have eased the landing did not deploy and the site was much harder than it looked like before (the site was chosen because there was supposed to be “soft” regolith). It bounced twice and almost float away completely. It had battery for 2 days which were of course not enough to conduct all experiments and it could not recharge because it was under some cliff. Nobody knew where it was and we could not identify pictures that it took.

Philae found

It puts me in awe to know that this picture is from a comet. (Philae sits in the right middle of the picture in shadow.)

Luckily Rosetta still orbiting the comet finally found it and put them all in context. The mission ends in 30th September and Rosetta will too crush on the surface.

close up of Philae

The picture of Philae

Now that is for some asteroid exploration back in time.

Three days back, 8th September OSIRIS-REx, an asteroid study and sample return mission was launched.OSIRIS-REx Mission Logo December 2013.svg

The last part is pretty huge, yes USA is for the first time going to return samples from an asteroid to Earth (Utah is the landing site).

It launched on the often used Atlas V and the whole mission for asteorid called Bennu will take 7 years. OSIRIS will stay on its surface for whole 505 days! (Look how planned this whole thing is!)

There are lot of instruments on its board which I wont go through all. There are many cameras because OSIRIS will first orbit the asteroid and scan its surface to find a good place to land.

It has special leg that will try to take samples using gas of nitrogen. It can take up to 2 kilograms and enough nitrogen for three tries.


Space NEWS #10 (Very close exoplanet)

today I am bringing news about the closest ever found exoplanet that is also potentially habitable. This planet is orbiting Proxima Centauri, the closest star to Sun.

This is great news. Like really, what is the probability of finding one of the best candidates for Earth like planet closest to us that it could get. But to be clear of what is really going on, it is not as that we are going to get a picture of it. Not at all, we do not even know its size (is probably above 1.3 of Earth’s) or anything about its composition. Its just that it is very very likely that the planet is there because of Doppler shifts and other fancy astronomical tools that enable scientists to discover exoplanets.

No, this is not how the planet looks like.. but yay! Random artistic pictures!

Proxima is red dwarf. This means that it is smaller and cooler than Sun. The difference is so huge that the planet may be in habitable zone even though it is probably only 7.3 million kilometers away compared to Earth’s 150 [1]. So if there is water it may be liquid but nothing is very sure. If there are some greenhouse gases it is probably warm enough.

Before leaving, just check out this cool comparison of the angular diameter (size) of Sun and Proxima from Earth and from the new planet (Proxima b):

Sun and Proxima compared

Yes, any life on Proxima b would have much bigger and redder star to look on.


Check out these two pages for more info: 1) 2)

[1]Proxima has surface temperature of 3050K, 0.1 percent of SOlar luminosity, radius 0.14 and 12% of Sun’s mass.

Juno has some real party instruments!

as I promised, today I will write about instruments that Juno has acquired for the journey to Jupiter. Also I wont post anything for something like two weeks again because I am going with my mum and sister to Poland on vacation. After that I will be few days at home and then I will go to Germany for one year (of course I will start writing again at that time).

Juno is very well prepared to gather some data, here are all the things that Juno is capable of:

Gravity measurements

To measure if Jupiter has solid core or not scientists are going to measure Doppler shift of radio waves transmitted back to Earth. The changes of gravity from computed should be from either storms if they go very deep into the atmosphere and/or changes of density and surface of the core if it exists.

JADE – Jovian Auroral Distributions Experiment

Those are three detectors that each covers 120┬░ + one special detector that has 270┬░ view. This experiment is trying to observe the auroras of Jupiter by measuring the charged particles that create them.

JEDI –┬áJupiter Energetic Particle Detector Instrument

Right this does not correspond to the acronym but you know.. Jedi ­čśë

This experiment is similar to JADE except that it consists of only one detector and detects particles with lower energy.

JIRAM –┬áJovian Infrared Auroral Mapper

Again this one watches over auroras but also it makes infrared images of the atmosphere.


This is somewhat unnecessary camera that is going to collect pictures for public. There was even voting for what pictures it should take because it wont have so much time. As I said in the last post it is going to have some cool resolution but we will have to wait about month for it.


Juno also has magnetometer that will measure the strength of the magnetic field and its other attributes. It is quite big instrument with 3.6 meters height.

MWR – Microwave radiometer

Such thing was not used before on Jupiter so it could be huge surprise what we will see in microwave radiation because that is exactly what wavelength this instrument measures.

Ultraviolet Imaging Spectrometer – UVS

This one will watch Jupiter in ultraviolet. Here nice target are again the aurora because they are much easier to watch in UV especially because you can do it even during day.


Waves are basically two antennas which are about 3 meters long and then one smaller electronic device. This instrument is going to measure the interactions between magnetic field and atmosphere. The smaller device is mostly wire, turned 10,000 times around some bar.

From all of this it could seem that Juno is going to measure only magnetosphere and auroras though this is simply what you can do without needing to crush into the planet. (Which will happen anyway though Juno wont survive of course). All of these things are quite observable from far away and yet they can tell you a lot about the planet.


Check out these pages for more info: 1) 2) 3)

Juno is right at the party!

wondering what to write about today I decided that best would be to catch up with the mission Juno which is going to explore Jupiter.

Juno Reaches Jupiter

This is doodle by Google which shows the excitement of the scientists as they watch the signal from Juno coming back after it started to orbit Jupiter.

So it has been basically 5 years since NASA launched Juno (2011 August). This satellite is the second one after Galileo that is going to orbit around Jupiter. Most of others were just on flyby to other places and Galileo kind of broke.

Artist’s impression of Juno.

Juno mission is going to last for about 1 and┬á┬Ż of a year. This seems kind of short time when you consider that it took alone 5 years to get there. SciShow Space said that it is because NASA does not want to risk getting Europa spoiled with ANY organisms from Earth though I think that this is nonsense and that NASA just does not have enough money which is something I will get to in another post.

What will we get? Well hopefully we will learn more about formation of Jupiter and whole Solar System, this is basically the main purpose but priority is also the gas of Jupiter and its magnetic field. We wont get probably any pictures of the moons because they are not part of the mission and they would be very small. At one point the JunoCam instrument will have a great resolution of Jupiter, about 15 kilometers per pixel. This is something amazing since Jupiter has about 140,000 km in diameter.

Right now Juno is on what is called “capture orbit”. Those are two 53.5 days long orbits which will then lead to 14 day science orbits where the real data is going to be harvested.

Everything about the mission is quite planned.

Juno is also the furthest man-made satellite that is powered only on solar panels. At the distance where it is, there is only 4% of sunlight compared to Earth.

Next time I will cover what tools Juno carries.


What does the 3rd Kepler’s law say?

today I want to do a short post about the 3rd Kepler’s law. I kind of really like it because it has very simple explanation but lot of uses at the same time.

The law goes as follow:{\frac  {T_{1}^{2}}{T_{2}^{2}}}={\frac  {a_{1}^{3}}{a_{2}^{3}}}

T stands for time and a for semi-major axis of ellipse, that is basicly radius for planets since

What is semi-latus rectum?

their orbit is highly circular. The index┬á1┬áand┬á2 stands for first and second object, basicly you are comparing two objects with each other though they must orbit the same body. This is very useful since you can compare anything in Solar System orbiting Sun with Earth. Why is it useful? Because Earth’s semi-major axis is 1AU and orbit lasts for 1 year which means that this fraction will disappear and you are left only with the object you want to calculate with.

Where did this even came from? The prove for this equation is very simple and basicly stands on the fact that centripetal force equals gravitational force for our orbiting object.


We can find the equations for both of these forces and from them finally get to the Kepler’s law:KeplerLaw3

Ok, before you start to freak out, this is completely easy. First line is clear, I have accidentaly indexed Fd instead of Fc because in Czech the force is called “dost┼Öediv├í”.

Second line shows the forces and their equations, third canceles the mass of the orbiting body and the radius of orbit. Since v=s/t we can write it down as is shown. Also watch out because┬ás is whole orbit so s^2=4¤Ç^2

The equation that you have in fifth and six line is also usable equation! It is more general and does not need the second orbiting body but it needs the mass of object. From this equation you can also figure out the mass of Sun which is completely amazing! (You have to watch out for the right units!)

After the small space I have divided the equation by the same one except that it works with some other object orbiting the same star (or planet..), with this step I will get easily rid of all the┬á¤Ç,┬ágravitational constant and mass of the center object.

Now we have the original 3rd Kepler’s law!


PS: in the prove we also assumed that r=a which means that planets orbit on circles not ellipses but it is accurate enough

Right ascension and declination

in the last post I explained what is celestial equator and what is ecliptic, check it out if you did not read it.

Today I want to look upon right ascension and declination which is the system that lets you describe where stuff is on the sky and it does not matter where you are on Earth.

Basicly the gif on the left explains it: you have Earth inside your celestial sphere. Through the middle comes the equator, that makes sense but since Earth is tilted by 23┬░ degrees to its ecliptic, in our picture it must be ecliptic that is tilted (it is always just a reference frame).

Of course there are two points where equator and ecliptic meet, as I said in the last post, those are autumn and spring equinox.

As you go around the circle your “value” gets higher, it is called right ascension (or RA). Full circle has 24 hours and when you take some place there it tells you in how much time the equinox that has “0 hours, 0 minutes” will get there.

The value of right ascension increases from west to east (’cause rotation).

Of course since we are in 2nd dimension we have to use another coordinate that will tell you how above you are from the equator (or below). This is called declination and is normally measured in degrees. Since furthest you can be from the equator are poles the angle wont be bigger than 90┬░.