My question pertains to the nature of the Islamic Calender and why we use the lunar calender to decide when Shahr Ramadan begins. 

In the Holy Quran Allah swt states in multiple ayats to use the Sun and Moon to calculate the years and months (Surah Yunus verse 5 for example being one of the obvious ones). Why then do we as muslim just rely on the moon? Why not utilise both celestial beings for calculation, does Allah swt not state "The sun and the moon are (bound) by a (fixed) calculation." (5:55).

Even in Surah Kahf verse 25, "300 years [add] 9" can be see as 300 solar years and/or 309 lunar years as 300 solar years is equivalent to 309 lunar years. 

Furthermore, many of the Islamic months correspond to seasons (such a Rabi being Spring or Jumada signifying low water levels), why then would a month be called spring if it moves around the year and doesnt always correspond to spring and why would a month be called Jumada which comes right after Rabi’ al thani if it doesnt correspond to the low rainfall levels which come right after the end of spring in the middle east? 

In additon, in Surah Baqarah verse 185 which is actually talking about Ramadan, Allah swt states "Allah wants ease and not hardship for you so that you may complete the number of days required". How does a shifting ramadan which forces some people in the northern hemisphere to fast sometimes 18+ hour days account for this ayat? How is it fair some fast 12 hours and some fast 18 hours? How is it fair and merciful that in some poor countries such as Iraq and Pakistan many of those who fast end up dying of thirst because the shifting ramadan ends up in the middle of summer. Also how can someone who lives in a Scandinavian country in which several months in the year have 24 hour daylight fast? 

It only makes sense that shahr Ramadan as well as all the islamic months are fixed and dont shift. When using a lunisolar calender this is the case, every 32 months an additional month is added which aligns the months back to their seasons. This is similar to how the west adds an additional day every 4 years to February.  By doing this, shahr Ramadan aligns closely to the autumn equinox (around september) this rectifies all the issues stated above as all countries have similar length days (12 hours of daylight approximately), even the northern most cities in the world. This also makes more sense as around this time middle eastern countries start to cool down and days aren’t as hot.

There are so many more proofs even from hadiths and the quran relating to the battles of badr and yarmouk and the weather conditions on those days and how if we use the lunar calender they dont correspond but if we use the lunisolar calender they do correspond. 

We follow the way the Prophet told us to. Our scholars are not making this up on their own. All Muslim scholars agree that the Islamic calendar is a Lunar calendar.

There is a famous hadith of the Prophet- Fast when you sight the hilal and break the fast when you sight the hilal.

وعن سعد ، عن العباس بن موسى ، عن يونس بن عبدالرحمن ، عن أبي أيوب ابراهيم بن عثمان الخراز ، عن أبي عبدالله ( عليه السلام ) قال : قلت له : كم يجزي في رؤية الهلال ؟ فقال : إنّ شهر رمضان فريضة من فرائض الله فلا تؤدوا بالتظنّي ، وليس رؤية الهلال أن يقوم عدة فيقول واحد : قد رأيته ، ويقول الآخرون : لم نره ، إذا رآه واحد رآه مائة ، وإذا رآه مائة رآه ألف ، ولا يجزي في رؤية الهلال إذا لم يكن في السماء علة أقل من شهادة خمسين ، وإذا كانت في السماء علة قبلت شهادة رجلين يدخلان ويخرجان من مصر .
[ 13440 ] 11 ـ وبإسناده عن علي بن مهزيار ، عن محمد بن أبي عمير ، عن أبي أيوب (1) وحماد (2) ، عن محمد بن مسلم ، عن أبي جعفر ( عليه السلام ) قال : إذا رأيتم الهلال فصوموا ، وإذا رأيتموه فأفطروا ، وليس بالرأي ولا

The Quran states that the months are 12

إِنَّ عِدَّةَ الشُّهُورِ عِنْدَ اللَّهِ اثْنا عَشَرَ شَهْراً فِي كِتابِ اللَّهِ يَوْمَ خَلَقَ السَّماواتِ وَ الْأَرْضَ مِنْها أَرْبَعَةٌ حُرُمٌ ذلِكَ الدِّينُ الْقَيِّمُ فَلا تَظْلِمُوا فِيهِنَّ أَنْفُسَكُمْ وَ قاتِلُوا الْمُشْرِكِينَ كَافَّةً كَما يُقاتِلُونَكُمْ كَافَّةً وَ اعْلَمُوا أَنَّ اللَّهَ مَعَ الْمُتَّقِينَ

Yunus 5
هُوَ الَّذِي جَعَلَ الشَّمْسَ ضِياءً وَ الْقَمَرَ نُوراً وَ قَدَّرَهُ مَنازِلَ لِتَعْلَمُوا عَدَدَ السِّنِينَ وَ الْحِسابَ ما خَلَقَ اللَّهُ ذلِكَ إِلاَّ بِالْحَقِّ يُفَصِّلُ الْآياتِ لِقَوْمٍ يَعْلَمُونَ

He is the One who has made the sun a glow, and the moon a light, and determined for it stages, so that you may learn the number of the years, and the calculation (of time). Allah has not created all this but for a rightful purpose. He elaborates the signs for a people who understand.

Allah does talk about the sun however the Quran also talks about the moon. The verse says it’s through the moon you can calculate time.

The Earth revolves around the sun and it takes one full year. It’s easier to keep track of the months using the moon rather than the sun because that’s a much longer circle.

So it is actually more difficult to rely on the sun alone. Furthermore even the solar calendar has leap years and some months are 29 days others are 30 or 31

We have an easy system where a month is either 29 or 30 days.

As for the solar calendar making it difficult to fast, scholars say if someone is in extreme difficulty they can break their fast, they can also travel. People have been fasting for 1400 years, it hasn’t been that big of a probleM.