8% isn't a bad click through rate, especially if you're sending newsletters weekly. I don't see anything about frequency next to the subscribe button on your homepage, so people may have been expecting something different, which would drive down the click through rate.
I've also seen enlight.ml posted here before, so my guess is quite a few of the subscribers came from HN or sites like it. Typically the users you get through these kinds of postings aren't all in your target audience. This might also be resulting in a lower click through rate.
The key to getting a high click through rate is delivering a lot of value. There aren't any good shortcuts.
Here are some things that may help you figure out and boost your perceived value:
* Send a survey to your users asking them what types of updates they want to get from you, and how often.
* Figure out if the same 8% engages each time, and what the patterns of click throughs are (do some people click through every single time?).
* See if anyone on your list would be willing to chat with you about the value they get from your newsletter/site.
* Do some content marketing/promotion to attract a more targeted audience.
* Make sure you're selling the benefits of doing the lesson in your emails.
* Have some clearer messaging around the signup box for the list.
I run a site to teach people data science, and I've learned a lot of marketing lessons along the way. Happy to chat further if you want -- my email is in my profile.