SKY-WATCHER'S DIARY: How to understand size of our solar system?

We all know about our solar system that there is a sun and eight planets are circling around sun. We usually see this picture (Fig.1) in our school textbook or in astronomy books. It hardly gives us idea of size of sun, planets and distance between them. Such kind of figures are given to just get idea about members of our solar system and not their size and actual distance from sun.

Fig.1

There are different kinds of representations are available to give size of our solar system. Scientists, artists and craftsmen have designed different kind of models as well as picturization of solar system. Here we can look at the figure 2 which is based on the size of the solar system.

Fig.2

In this figure 2 size of Oort cloud which is within the extreme limit of the sun’s gravitational influence. Oort cloud is around 1 lakh Astronomical Unit far from sun. 1 astronomical unit is the distance between sun and earth. At the center of this Oort cloud a small dot is region of Kuiper belt region where small objects including Pluto exist. At the center of this part is solar system with some outer planets orbits. Then at last it magnified with the inner solar system region.

There is one more image which can give idea of size of planets with respect to sun

Fig.3

We can see in the figure 3, comparison of planets with sun. Our planet is around 109 times smaller than sun. It even significantly smaller than gas giant planets like Jupiter, Saturn, Uranus and Neptune. So, one can imagine how insignificantly small is our city, our home and ourselves in front of this gigantic bodies. If we increase size of our planet to the size of sun, then flight from Delhi to Istanbul will take almost a month to travel. Usually, this flight takes around 7 hours. Our sun is 109 times bigger than earth then 109 x 7 = 763 hours. I am assuming that earth size is like sun and speed of flights remains in present scenario.

Relative size and distance of planets comparing with sun

Object	Diameter (KM)	Relative to Sun	Orbit Radius (KM)
Sun	13,91,990	1.000000	--
Mercury	4,866	0.003495	57950000
Venus	12,106	0.008696	108110000
Earth	12,742	0.009153	149570000
Mars	6,760	0.004856	227840000
Jupiter	1,42,984	0.102719	778140000
Saturn	1,16,438	0.083648	1427000000
Uranus	46,940	0.033721	2870300000
Neptune	45,435	0.032640	4499900000
Pluto (Minor Planet)	2,274	0.001633	5913000000

Here we can see the orbit radius which is distance of planet from the sun. Its quite vast and numbers to denote these distances are also big. So, to minimize numbers we convert this distance to more user friendly unit where we can easily write and remember the numbers. We use unit like Astronomical Unit which is distance between sun and earth which is roughly 150 million km. Shortly it is written as AU. When we measure distances outside the solar system we use unit Light Year. One Light Year is a distance travel by light in one year which is around 9.5 trillion km. Here in figure 4, distances in solar system showed in Astronomical Unit and also time required by light to travel these distances.

Fig.4

How to give idea of scale of solar system to students

There is not any picture is available which can give clear idea of the scale of the solar system. Above images are best but they also having some factual errors. Like figure 1, 2 and 3 planets orbits are not clearly giving idea of distance of planets from each other. If we go in to the space and have a look at our solar system, the distances between planets are enormous that one can not even see nearest planet. Distance planets brightness will decrease as their distance increase from the sun.

There is an activity we can do with students on giving how big our solar system is. In this activity one can make actual scale size model of solar system. Before giving idea of the scale of the solar system students must have skill of converting figures to the proportional scale or teacher can help students in this part. So, we can make proportional model of our solar system. For this activity we required a huge ground, at least 4 km wide. But we can not get usually such wide field. So, within half to one kilometer will also be enough for getting idea.

We can use clay to make models of sun and planets. We can take conversion unit as so we can build sizable planet models. If we consider size of sun is one meter, then we can make models size and distance as per below

object	Object size in mm	Distance in meter
Sun	1000	-
Mercury	3.4	41.633
Venus	8.6	77.67
Earth	9.1	107.457
Mars	4.8	163.689
Jupiter	107.7	559.048
Saturn	83.6	1025.217
Uranus	33.7	2062.145
Neptune	32.6	3232.919
Pluto	1.6	4248.15

Teachers can make clay models of planets. One can use different color for making different planet

Sun – Yellow or Orange

Mercury – white or grayish

Venus – Yellow

Earth – Blue

Mars – Red

Jupiter – Yellow with different color of strips on it

Saturn – Pale Yellow

Uranus- Blue

Neptune – Blue

Pluto – Grey

These colors are just for representations.

After making clay models teachers can give one clay model to each student. Then in open field ask students to measure distance from Sun with measuring tape. At the proportionally converted distance students will stand according to order of planets. Ask students who are holding to these planets to see at each other models. Then takes their reactions on it.

Time in Solar system

We have seen that planets orbit around the sun in different time depends on the distance from it. We called it planets revolution period. Planets also rotate on their own axis we called rotation period or a day of planet. Scientists have designed models for showing their different orbiting time. This instrument is called as Orrey. This instrument is designed with many gears and wheels to synchronize duration of different planets orbiting time. One can see it in local science Museum if they have it. Long back, I have seen one in Mumbai planetarium.

Fig.5

Here it is rotation and revolution time of planets with respect to Earth.

Planet	Rotation Period or a Day	Revolution period or a Year
Mercury	58.6 days	87.97 days
Venus	243 days	224.7 days
Earth	0.99 days (23 Hrs. 56 Min.)	365.26 days
Mars	1.03 days (24 Hrs. 37 Min.)	1.88 years
Jupiter	0.41 days (9 Hrs. 55 Min.)	11.86 years
Saturn	0.45 days (10 Hrs. 40 Min.)	29.46 years
Uranus	0.72 days (17 Hrs. 14 Min.)	84.01 years
Neptune	0.67 days (16 Hrs. ^ Min.)	164.79 years
Pluto	6.39 days	248.59 years

We can see from above table that Mercury’s a year is equal to Earth’s three month. A day on Venus is bigger than its year. Mars and Earth have almost similar day length. Mars takes almost Earth’s two years to complete an orbit around sun. As distance increase planets Revolution period increases. You can understand it by other way also that if you go on Jupiter your birthday will come after every 12 years of Earth. If you go on Mercury, you can celebrate your birthday after every three months of Earth. You can notice that giant planets Jupiter, Saturn, Uranus and Neptune have short day length compare to Earth. It means that a year on Jupiter is not only the equivalent of 4,333 Earth days, but 10,476 Jupiter days. If you on Jupiter, you see 10,476 sunrises in a year.

Models in science

We have just seen how our solar system is big on scale of size distances and time. If we are teaching about solar system in science classroom then we can use different models to give idea of the solar system. Why we care about different models? There are many scientific concepts and phenomena which either not directly observable or hard to explain. The representation of such scientific concept and phenomenon in real world is very problematic. To understand such concept, we require models. It similarly applies to the concept of solar system too. After doing basic observation of planets with naked eye teacher can work on how our solar system can be map on the scale of size, distance and time. To construct this concept in students mind we requires multiple models.

We can see in science there is no single model which can represent an entire phenomena or concept. Different models at different level helps to understand the concept. So, teachers can decide at which level he or she can take help of model. Teachers also should know how models are built. Models are based on different kind of data which is collected or derived based on direct observation or mathematical calculation. (In this case of solar system model data collected is based on direct observation as well as mathematical calculation which are taken from different sources). It is also important to know that models should not be treated as the reality or the theory or concept itself, but they should be perceived as kind of simplification or clarification of the concept. Science education aim is to help students constructing knowledge so teacher must ensure that the models they use in science classroom must stimulate imagination, discussion, investigation and explanation in learning process.

References

Calculating Cosmos by Ian Stewart
Why Models are advantageous to learning science, by Gail D. Chittleborough and David F. Treagust, in Educacion Quimica, Volume 20, Issue 1, January 2009, Page 12-17
The Importance of Modelling in Science Education and in Teacher Education, on Researchgate.net, by Yasemin Godek, Ministry of Youth and Sport, Turkey, January 2004